Cut tops off tomatoes. Using a teaspoon, carefully remove, and reserve, the pulp. Force pulp through strainer, rendering 1/2 cup of juice.
Lightly sprinkle pepper inside the tomato shells. Invert on paper towels, and let drain.
Over low heat, melt 2 Tbsp of butter in skillet. Add the garlic and onion. Sauté until soft, about 5 minute; set aside.
Combine onion and garlic with the reserved tomato juice, spinach, sour cream, cream cheese, mozzarella, Parmesan, basil, and bread crumbs in a food processor. Blend thoroughly.
If mixture appears too thin, gradually add more bread crumbs, if it's too thick, add more tomato juice. Refrigerate 30 minutes.
Preheat oven to 350°F Scoop the filling mix into the tomato shells. Top with more bread crumbs and dot with bits of the remaining butter. Place in lightly greased baking dish and bake for 20-30 minutes.