The correct way to slice an onion: 5 cooking rules explained

In the kitchen there are unwritten rules we follow: never drop potatoes into boiling water, alternate between wet and dry ingredients when mixing batter, press tofu before tossing it in a pan. But what's the science behind these rules? Tim Cebula, senior food editor at Cooking Light and co-author of "Why You Dredge, Rest, Pulse, and Process," explains.

1. Never drop potatoes into boiling water
 

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Sally Swift: My mother drilled a rule into me over and over again about boiling potatoes. She said you always have to start the potatoes in cold water, never drop potatoes into boiling water. Is that true?

Tim Cebula: Absolutely. You wouldn’t think the potato would require such TLC, but it does.

Because it’s a little bit denser than a vegetable like green beans, for instance, the density means that it’s going to take a little while longer for the inner temperature to match the exterior. If you drop a cubed or quartered potato directly into a pot of boiling water, what you’re going to end up with is a potato that’s mushy, gluey and crumbly on the outside and practically raw on the inside.

If you start it in cold water, the water begins to raise its temperature gradually, giving the potato a chance to raise gradually with it. Then you’re going to get a much more evenly cooked potato.

2. Preheat the pan before you roast vegetables
 

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 SS: What about the rule about preheating pans in the oven before you roast vegetables? Is that something we need to follow?

TC: It absolutely is. But it’s something that a lot of us don’t even think to do, while oddly we’re all very aware of how important it is to preheat your pan on the stovetop before you add things in. The same principle applies here: When you preheat your pan for roasted vegetables, you’re going to end up with less sticking and better browning.

This is particularly important with tender spring and summer vegetables-- if you start them in a cold pan, it’s going to take that much longer in the oven while they roast for them to cook. You’re probably going to end up with mushy vegetables that don’t get much browning.

But if you preheat the pan and you lightly coat your veggies in oil, the minute they hit the pan you’re going to hear that sizzle just like you should when you sauté them.

SS: You get that sear. Does that work with proteins too?

TC: It would work with proteins; the difference though is when we’re roasting proteins, we often tend to brown them on the stovetop first. But if you don’t brown them on the stovetop for whatever reason, it’s a great idea to get your roasting pan good and hot and then put the meat inside.

SS: It would speed things up too.

TC: Absolutely. We can all use a little extra time.

3. Cut onions vertically if you want them to hold their shape
 

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SS: People have so many opinions about what the correct way is to slice an onion. Will you tell us once and for all what the difference is between slicing around from the equator, or slicing from pole to pole?

TC: It’s a polarizing topic and people feel really strongly about it. Slicing from pole to pole, or vertically, is essentially to slice along with the fibers of the onion. If you peel an onion, you will see that there will be green lines that run vertically from pole to pole. When you slice with these fibers, the onion pieces you end up with tend to hold up a little bit better as they cook.

SS: So if you want onions to hold their shape, you want to cut them pole to pole?

TC: Exactly. This is great when you’re long-cooking onions, like a caramelized onion. If you want the onions to be separate and not mushed together in a jam, that’s the way to do it.

But then again you might want them to mush together in a jam and that’s fine. That’s when you would cut them in half moons or across the equator, and then they’re going to break down a little bit more.

SS: Dicing would do it even further, right?

TC: Exactly.

4. When mixing a batter, alternately add wet and dry ingredients
 

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SS: What about this rule when you are baking and you are mixing a batter -- the instructions come back again and again to alternate the dry ingredients with the wet ingredients. Is there real science behind that?

TC: There is a method to that madness. It’s never really fully explained in recipe methods, but it’s important to follow because what it comes down to is bubbles. When you’re mixing up a cake batter, it’s the bubbles, unpopped and whole, that will give you the tender open crumb in the cake that you want. When you’re beating sugar into softened butter or when you’re beating eggs into a frothy foam, you’re getting these bubbles.

To gradually add the dry ingredients is going to keep the bubbles from popping; that’s what you want for a nice light, airy cake. Dump and stir is fantastic for something that’s supposed to be more dense, like a brownie or something like that. But for the light, airy cake that you want, you’ve got to take a little bit of care and keep those bubbles intact.

5. Press tofu to achieve a golden crust
 

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SS: More and more people are eating tofu. Often you’re told to press it to get some of the water out. Does that really do anything?

TC: It sure does. Often a lot of the tofu you’re going to get, the firm and extra firm, comes packed in water. You press it between paper towels and press it with a heavy skillet or something for about 20 minutes to get the water out.

Water, as we know, is the enemy of brown and crisp food. If you want to sauté these tofu cubes, tofu planks or however you slice them and get a nice golden crust on them, you’ve got to get that water out of there or else they’re just going to steam in the pan.