Preheat your oven’s broiler. Place the mushrooms (gill side facing up) on a baking sheet. Drizzle them with 2 teaspoons olive oil, and sprinkle on a pinch of salt and pepper. Put them into the oven and broil for 5 minutes. Flip the mushrooms over (so the gill side is facing down) and broil for 5 minutes more. Remove the mushrooms from the broiler and preheat oven to 425 F.

Heat the remaining 1 tablespoon olive oil in a medium skillet over medium heat. Add the onion and sauté until softened and just starting to brown, about 5 minutes. Stir in the spinach, garlic and thyme and cook until the spinach is wilted, about 1 to 2 minutes. Remove from the heat and allow to cool completely.

In a medium sized bowl, mix together the spinach mixture, ricotta cheese, egg, half of the shredded mozzarella, 1/4 teaspoon salt and a pinch of pepper.

Place each mushroom (gill side facing up) in an individual gratin dish. Divide the spinach/ricotta mixture between the 2 dishes and top with the remaining mozzarella.

Bake at 425 F for 10 minutes. Then turn the oven to broil and broil the mushrooms for a couple minutes to brown the tops.