Preheat the oven to 350 degrees F.
Cut the chiles in half lengthwise and remove the seeds. Arrange in 1 layer on the bottom of a buttered 13 by 9-inch baking dish. Cover with the cheese. Whisk the eggs with the sour cream in a large bowl until blended. Season with salt and pepper. Pour the eggs over the cheese, smooth the top, and bake in the middle of the oven until golden, about 45 minutes. Serve with salsa, sour cream and guacamole on the side, if desired.
Coarsely mash 1 peeled, pitted ripe avocado, preferably Hass. Add some fresh lime juice, chopped cilantro leaves, chopped cherry tomatoes, 1 small minced garlic clove, 1/2 serrano chile, or jalapeno, minced, and kosher salt, to taste.
Recipe courtesy Sara Moulton, Sara Moulton Cooks at Home, Broadway Books, 2002