This is my favorite new weekend breakfast/brunch item. I like frittatas because in spite of their fancy rep, they're deceptively easy. No flipping is required with this one (I often break my omelets that way!), and it puffs up beautifully. I skipped the mushrooms, not being a fan of them. I also substituted the sausage for spinach once for a friend who is a self-described "mostly vegetarian," and she loved it.
After some experimenting, I discovered it can easily be converted to a recipe for two, in case you don't have a crowd to cook for.
I simply use the smaller pan on the right (this one's about 6 inches across the bottom and 8 across the top), use 5 eggs instead of 9, halve the other ingredients, and cut the oven time down to 10 minutes. And it still comes out perfect.