2 parts pepper to 1 part onion. I used 4 New Mexico Green chiles (long thin pale green hottish peppers). Traditionally, you'd use Poblanos, aka Ancho peppers (those dark green squatish triangular peppers.) You could use jalepenos, but bear in mind that this shouldn't be blazing hot, so either pray they're mild, or use less.
Pop a pan on your oven or prep your broiler or grill to fry your peppers. You want the outside to be blackened, but not to char the entire pepper. Sometimes, using cold peppers from the fridge makes broiling them more easily. You're looking for color changes in the skin, a lovely roasting pepper aroma, flexibility in the pepper, and the skin to kind of blister. Pop the peppers in a paper bag, plastic bag, in a bowl covered with a chopping block, etc. so they can steam.
I used a 1/4 of an onion, since I didn't want this to be too sweet. You can use more. Slice it into thin crescents and pop it in a pan with a bit of oil or butter or lard, as your conscience dictates. Fry till clear. Meanwhile, peel the skin off your peppers (don't worry if you can't get all of it.) Open the peppers by hacking off the top and then cutting a slit down the side. Remove seeds and ribs to cut down on heat. Chop in 1/4 inch strips, cutting these in half if too long. Toss them in the pan. Salt lightly, and add a pinch of oregano or garlic if you'd like.
Dollop on about 1/4 cup of crema, yogurt, cream fraiche, or sour cream thinned with some milk. You want to coat the pepper/onion mixture. Just heat enough to take the chill off the milk product. Serve with tortillas or with other materials to make tacos.
(Leftovers? Blend till smooth, and then thin with water or broth for a soup.)