Set the oven to 425*F. Drizzle a roasting pan with 1 tbls. of olive oil and add tomatoes, 1/2 teaspoons salt, 1/2 teaspoons pepper, and basil. Toss to mix. Roast for 15-20 mins., or till tomatoes start to burst. Keep the oven hot.
Preheat the grill or broiler to medium heat. Place the mushrooms, gill side up, on cutting board(or work surface) and sprinkle with 2 tbls. olive oil, balsamic vinegar, a little salt, and remaining 1 teaspoons pepper. With remaining 1/2 tbls. oil, lightly coat the rounded side of the mushroom caps.
Place mushrooms, gill side down, on grill or broiler pan. Grill or broil for 6-8 minutes on each side, until soft and slightly charred.
Move the mushrooms to a roasting pan and fill with 10-15 tomatoes each. Top with the cheese. Place in a hot oven until the cheese bubbles. Place each mushroom on a mound of steamed spinach and serve up immediately. Enjoy!