To roast chiles, use fresh New Mexico or Anaheim chiles, or the darker green, thicker-skinned poblano peppers (often more readily available). Poke whole chiles with a fork to keep them from bursting. Place on a baking sheet and roast 4 to 6 inches from the flame of a preheated broiler, or char directly over the open flame of a grill or on a hot comal, turning occasionally for even blistering and light charring.
Place the charred chiles in a brown paper bag to steam for 10 minutes, then peel away the charred skin. Don’t rinse the chiles under running water or you’ll lose flavor! Make a slit in one side of each chile, leaving the stem intact. Remove some of the seeds and the chiles are ready for stuffing.
To make the chile rajas (green chile strips), remove the seeds and stems from the roasted chiles and cut into 3-by-?-inch strips. Use in chile con queso and other recipes.
Heat the butter and oil in a pan. Add the onion, garlic, jalapeños, rajas and salt and cook for about 5 minutes—tossing well. (This may be made in advance and reheated.) Lower the heat, add the cheese and any optional ingredients and stir gently until melted. Serve immediately in a warmed bowl.