Featured as our April 2018 Recipe of the Month, this has become a favorite gluten-free alternative to the quiche. We use lots of baby belle mushrooms, which then resembles a crust. And lots of cheese, lots!
Preheat oven to 350° F.
In a large ovenproof skillet over medium heat, melt butter. Add mushrooms, onions, and garlic, and saute until tender, 5 to 6 minutes.
In a medium bowl, whisk together eggs, milk, mustard and seasonings. Pour over vegetables in skillet.
Arrange tomato slices on top and sprinkle with cheeses.
Transfer to oven and bake until eggs are set, about 25 minutes.
Let stand for 5 minutes.
Cut into wedges and serve from skillet, or slide the whole frittata onto a warm plate.