Shiitake Mushroom and Cheese Frittata

Preheat oven to 350 degrees. Heat oil in a 9- or 10-inch ovenproof nonstick skillet over medium heat until hot but not smoking. Add mushrooms, onion, and thyme; cook, stirring, until mushrooms and onion are golden brown, about 10 minutes. Stir in 1/2 teaspoon salt; remove skillet from heat.

Whisk eggs in a large bowl until frothy. Stir in the ricotta, 1 teaspoon salt, and pepper to taste. Place reserved skillet over medium heat. Immediately pour in egg mixture, and lightly stir to distribute mushrooms and onion evenly. Reduce heat to medium-low; cook 2 minutes, drawing cooked egg away from sides with a rubber spatula to let uncooked egg flow underneath. Cook until just starting to set, about 2 minutes more.

Transfer skillet to oven. Bake until frittata is just set and top is pale golden, 10 to 12 minutes. Serve frittata in skillet, or slide onto a serving platter.