Diana's recipe from "The Essential Cuisines of Mexico"
3 md Tomatoes (1 pound), broiled
2 ts Peanut or safflower oil
1/4 md Onion, finely chopped
1 The mashed tomatoes
1/4 ts Salt
1 Whole chile habanero, lightly charred
Remove any very blackened pieces of the skin from the tomatoes and mash the tomato roughly. Heat the oil and cook the onion, without browning, until it is soft. Add the mashed tomatoes, salt, and chili. Cook the sauce for about 8 minutes over a medium flame until it has reduced and thickened.
The recipe does not state this, but I found the Habanero flavor to be a bit light, so I think I will half the pepper next time, rather than leave it whole.