If you like a crusty top, sprinkle some dry cheese like parmesan or manchego, or a combination of one of these cheeses and a few panko bread crumbs on top of the frittata right at the end of the cooking time. (yes-moreplease.com)
Place skillet under broiler and cook until top of frittata is golden brown, 2 to 3 minutes. (vegetariantimes.com)
Sprinkle cheddar over top of frittata and broil about 4 inches from heat until cheese is bubbling, about 1 minute. (bakinandeggs.com)