Step 1

Adjust oven rack to its topmost position and set oven to broil.

Step 2

Place chiles skin side up on a baking sheet, and broil until skins blister, about 5 minutes. Be careful not to let them burn. Remove chiles from the oven and place in a glass bowl. Cover with plastic wrap and let chiles steam until cool. Remove and discard the skins.

Step 3

Preheat oven to bake at 350 degrees F (175 degrees C).

Step 4

Heat olive oil in a small skillet over medium heat. Stir in sliced onions, and cook until soft, about 5 minutes. Add the tomatoes and cook until onions and tomatoes achieve a sauce-like consistency, about 15 minutes. Stir occasionally. Season with cumin, stir briefly, then stir in the sour cream; remove from heat.

Step 5

To assemble the chiles rellenos: Slice cheese into 20 long rectangles (or sticks) 1/4-inch thick and approximately as long as the chiles. Roll each cheese rectangle into a chile half. Place rolled chiles into a baking dish and top with the tomato sauce. Crumble any extra cheese on top of the sauce. Bake for 30 to 40 minutes until the sauce is bubbly.