Heat oil in a frying pan and start to cook the tomatillos and jalapenos for about five minutes. Turn them over every one or two minutes. After five minutes, add the onion and garlic. Cook another five minutes, turning often. You want them to still have a golden brown sear, but a little bit of blackened skin is okay too. After ten minutes, turn off the burner and let it all cool down for at least five minutes.