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Don't add the oil yet; just heat your cooking implement. If you don't have a wok, use a heavy skillet with high sides. This type of skillet will keep the vegetables hot and let you stir them without spilling. Do not let the wok or skillet overheat, or you could start a fire when you add the oil. The wok or skillet is ready when a drop of water vaporizes within 2 seconds. Open any windows and turn on the oven fan if you have one. Stir frying can produce a lot of smoke and heat. Ideally, you should use an oil that can be heated to very high temperatures before it starts smoking. Peanut, canola, corn, safflower, and rice bran oil are all good choices. Don't use extra virgin olive oil, sesame oil, or butter, since these will smoke too quickly in the high heat. Hold the handle of the skillet or wok and rotate it so the oil coats the whole surface. It should break apart into a chain of oil beads and easily run across the bottom of the pan. If the oil is slow to spread, the skillet or wok probably isn't hot enough. Heat it until the oil is drippy before you start adding ingredients. Otherwise, the stir fry could come out soggy. The oil will begin to shimmer shortly before it smokes. That shimmer tells you the ideal time to add the first ingredients. If you don't catch the shimmer, add the ingredients when the oil starts to smoke a little. Now is the time to add garlic, ginger, scallions, and chili peppers, which will flavor the oil in preparation for the vegetables and protein. Use a wooden spoon to quickly stir the ingredients, or toss them in the oil if you can do it without spilling anything. Cook the aromatics for about 30 seconds before moving on to the vegetables and protein. Don't wait too long, since garlic and other aromatics burn easily in a hot wok. Besides proteins like tofu or meat, now is the time to add hard, dense vegetables like potatoes, broccoli, cauliflower, squash, and string beans. Stir the ingredients quickly with a wooden spoon, or toss them with tongs. To avoid making the stir fry soggy and unevenly cooked, only use enough vegetables to cover the base of the wok or skillet. Since a stir fry only takes a few minutes, you could cook your vegetables in batches, letting the wok and oil warm up in between. If the ingredients seem to be overcooking, stir more vigorously instead of turning down the heat. This keeps vegetables hot and dry, which results in proper stir fry cooking. Continue cooking the meat and denser vegetables until the meat is mostly cooked and the vegetables have become bright and slightly soft. This should take anywhere from 3 to 10 minutes, depending on the exact ingredients you're using. Once the bulkier ingredients are mostly finished cooking, add the vegetables that don't need much time to cook. Keep stirring vigorously as you add the remaining vegetables. Vegetables to add at this point include bok choy, bell peppers, and mushrooms. Ingredients that take even less time include zucchini, shredded cabbage, snap peas, and leafy greens. These can be added at the same time for simplicity, or you can wait until the other vegetables are nearly ready. Toss it in so that it coats all the other ingredients, then cook for an additional one to two minutes. Your stir fry should be nearly done after an additional 12 minutes. Pour the sauce in a line on the side of the skillet or wok, not the base, to keep the bottom of the wok hot. Don't use too much sauce, since it can make the vegetables too wet. It's texture is best hot off the wok. As soon as the sauce is on the vegetables, turn off the heat and transfer the food onto plates. The stir fry will be most delicious and tender right away, so don't let it get cold before eating. Steamed rice of any variety goes well with stir fry and soaks up the sauce nicely, but stir fries can also be delicious plain.
Heat a wok or skillet over high heat. Add two or three tablespoons of oil. Stir in the aromatics when the oil begins to shimmer. Stir in the ingredients with long cooking times. Add the vegetables with shorter cooking times. When the vegetables are tender, add a few spoonfuls of sauce. Serve the stir fry immediately.