Combine two tablespoons olive oil and clove of minced garlic in a small bowl. Brush mixture on inside and outside of portabella mushrooms. Sprinkle with salt.
In sauté pan, heat remaining olive oil on medium heat. Sauté spinach until wilted. Spoon half the spinach into each mushroom, and top with diced tomato followed by mozzarella.
Place filled mushrooms on cookie sheet sprayed with nonstick cooking spray. Bake for 15 to 20 minutes until cheese is melted and beginning to bubble. Mushrooms should be soft. Drizzle with balsamic vinegar and sprinkle with chopped basil.