Preheat oven to 350°. Cut chilies in half lengthwise and remove seeds. Arrange in 1 layer on bottom of buttered 13 x 9-inch baking dish. Cover with cheese. Whisk eggs with sour cream in large bowl until blended. Season with salt and pepper. Pour eggs over cheese, smooth top, and bake in middle of 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. Add some fresh lime juice, chopped cilantro leaves, 1 small minced garlic clove and kosher salt, to taste.