1. The Mushroom Treatment: Remove and discard the mushroom stems, and wipe the caps clean with a damp paper towel. Place a large, heavy skillet over medium heat for about 2 minutes. Add the olive oil, wait about 30 seconds, then swirl to coat the pan. Place the mushrooms cap side down in the hot oil, and let them cook undisturbed for about 10 minutes. Turn them over and cook on the other side for 10 minutes, then flip them over one more time and cook for 5 to 10 minutes more on their cap side once again. Leave them in the pan and set aside.

2. Core the tomato and gently squeeze out the seeds through the opening. Cut the tomato and the bell pepper into very thin slices.

3. Preheat the broiler. While it is heating, place the mushrooms cap side down in a Pyrex or ceramic baking dish. Top each mushroom with a few spinach leaves, some slices of tomato and bell pepper, and pinches of dried thyme, salt, and pepper. Take your time covering the top with grated cheese, tucking it into every crevice and cavity and trying to cover everything (including the edges of the mushrooms, so they won’t bum under the broiler).

4. Broil until the cheese melts and is turning golden. Watch carefully, as this will take only about 5 minutes or less. Serve hot, warm, or at room temperature, sprinkled lightly with red pepper flakes, if desired.