Need to feed a group for breakfast? Don't fuss over making omelets to-order. This couldn't-be-easier frittata is a total crowd-pleaser!
On sheet pan, toss first 4 ingredients; season. Roast at 450° until softened, 8 to 10 minutes. In large bowl, beat eggs. Stir in watercress, 2 tsp. salt, and 1 tsp. pepper; pour over mushrooms. Scatter cheese on top. Bake until just set, about 11 minutes.