Tomatoes and Goat Cheese: Preheat oven to 400°F (204°C). Place tomatoes in a large casserole dish. Drizzle on 2 Tbsp of the oil and the salt + pepper. Toss to evenly coat. Form a space for the goat cheese in the middle of the pan and set it in, topping the goat cheese with the pepper flakes.
Mushrooms: On a separate pan, place portobellos stem side up (you can remove the skin/gills if desired, but it's not necessary). Brush the gill-side with remaining 1 Tbsp of oil.
Bake: Place the pan with the tomatoes in the oven and cook for 30 minutes, or until tomatoes are soft and some have burst. After 10 minutes of cooking, add the pan with the mushrooms to the oven and cook for 20 minutes, or until tender.
Assemble: Stir the goat cheese and tomatoes together to liquify the gooey goat cheese. Add in basil, spinach, and garlic, stirring until spinach has wilted down a bit. Spoon tomato mixture over each mushroom and serve warm.