2 tablespoons extra virgin olive oil
1 (10-ounce bag) presliced mushrooms
Salt and freshly ground pepper
1 to 2 garlic cloves (to taste), minced
1 (6-ounce) bag baby spinach, rinsed
1/4 cup grated Parmesan or mozzarella
2 tablespoons low-fat milk
Heat 1 tablespoon of the olive oil over medium-high heat in a heavy 10 or 12-inch nonstick skillet and add the mushrooms. Cook, stirring or tossing often, until they begin to sweat and soften, 3 to 5 minutes. Add salt, pepper and the garlic, and stir together until the garlic is fragrant, 30 seconds to a minute. Add the spinach, a handful at a time and stir until wilted. Turn up the heat and let any excess liquid evaporate from the pan, then remove from the heat. Taste and adjust seasoning.
Beat the eggs in a large bowl. Stir in salt and pepper to taste, the milk, and the mushroom and spinach mixture. Add the Parmesan.
Heat the remaining olive oil over medium-high heat in the skillet until it feels hot. Pour in the egg mixture, scraping all of it in with a rubber spatula. Swirl the pan to distribute the eggs and filling evenly over the surface. Shake the pan gently, tilting it slightly with one hand while lifting up the edges of the frittata with the spatula in your other hand, to let the eggs run underneath during the first few minutes of cooking. Once a few layers of egg have cooked turn the heat down to low, cover (use a pizza pan if you don’t have a lid that will fit your skillet), and cook about 10 minutes, shaking the pan gently every once in a while, but making sure it cooks through.
Remove from the heat, and cool a bit.. Loosen the edges with a spatula. Carefully slide from the pan onto a large round platter. Cut into wedges. Serve hot, warm, room temperature, or cold.