New Mexico is justly famous for its Hatch green chiles, which are roasted and usually cooked into a piquant sauce that is used for every meal of the day. Fresh Hatch chiles are becoming increasingly available across the country, but if you can’t find them, canned ones are easily found at most grocery stores. Two 7-ounce cans will yield just about 1½ cups.
Cook the onion in the vegetable oil in a heavy saucepan over moderate heat, stirring occasionally, until softened, about 8 minutes. Add the garlic and cook, stirring, for 1 minute. Add the flour and cook, stirring constantly, for about 2 minutes. Stir in the chiles, then add the water, salt, cumin, and coriander. Bring to a boil, then reduce the heat and simmer gently, stirring occasionally, until thickened but still pourable, about 15 minutes. Set aside, covered, to keep warm.
NOTE: Green chile sauce keeps, refrigerated, for up to 5 days. It can also be frozen for up to 1 month.