Preheat oven to 350. Spray a baking dish with nonstick cooking spray. Place each portabella, gill side up, in the baking dish. Place 2 basil leaves on each. Top with a tomato slice. Lightly salt. Bake for 20 minutes. Top with cheese and chopped basil leaves, divided between the four mushrooms. Bake another 5 minutes or until cheese is melted.
You can use less basil, but I love it so I use alot!