There’s a saying in Peru that the corazón, heart, of the onion is what makes you cry.
I can tell you from experience that the heart (the core of the onion) has the same compounds as the rest of the onion that cause your eyes to tear up. The core does have a more concentrated flavor and often isn’t as sweet, so I usually remove them anyway and save them for stock.
What really will make you cry is a dull knife. A sharp knife doesn’t break down as many of the onion’s cell walls, so all of the juices stay inside the onion. If you aren’t used to chopping a lot of onions at once, the secret is to chop them very slowly, not crazy fast like you see chefs do on television. Concentrate on your job and slice the onions so the pieces are all the same size. I also never use a mandoline to finely shred onions. It breaks down the cell walls even more, so you lose the crunchiness that makes salsa criolla and so many other dishes with fresh onion so good. If you chop the onions and hour or so ahead, dunk them in ice water for a minute or two, then shake them dry with your hands to wake them up and get the crunchiness back.
Trim both ends off 1 large red onion, peel off the skin, and halve the onion from the root to the stem end (never through the middle). Use your fingers or a knife to pop out the interior cores (usually all white, with very little purple on the edges) and save them for stock.
Place one half, cut side down, on a work surface with your hands curled on top so your knuckles are very close to either end. Slice the onions as thick or thin as you like (¼ inch thick is good for salsa criolla), but try to make the slices all the same thickness.
Use the onions immediately, or refrigerate them for up to 1 hour. If refrigerated, transfer the onions to a bowl of ice water to crisp up for a few minutes. Drain and shake the onions a few times over the sink or a bowl to remove the excess water.
Follow the instructions above for halving the onion and removing the cores. Use a very sharp knife to slice the onion as thinly as you can. Move the knife blade Japanese-style through each onion half in one clean motion, from the handle end toward the top, like you are slicing fish to make tiraditos (see page 92). You should be able to almost see between the slices. When slicing onions a la pluma, be sure to very finely chop all of your other ingredients like tomatoes and cilantro, so they lightly cling to the shaved onions when you toss everything together. Makes about 2 generous cups.