Heat oven to 400 degrees F. Coat a 13 x 9 x 2-inch baking pan with nonstick cooking spray.
Trim thin slice from bottoms of tomatoes so they sit level. Slice each tomato horizontally into 4 equal slices. Drain slices on paper towels.
Combine in food processor or blender: ricotta cheese, basil leaves, mozzarella, Parmesan, egg yolk, bread crumbs, flour, salt and pepper. Process until smooth.
Spread 2 teaspoons cheese mixture on each tomato slice, except the top slice of each tomato. Stack the cheese-covered layers for each tomato, topping each with the plain tomato slice. Insert 2 toothpicks vertically into the top of each tomato to secure the layers together and prevent them from slipping during baking. Transfer to prepared baking pan.
Bake in 400 degree F oven for 15 minutes or until the skin just begins to wrinkle on the tops of the tomatoes.
Meanwhile, puree together the olive oil, fresh basil leaves, lemon juice and salt in a small food processor or blender. Let stand for 5 minutes. Strain and discard solids. Drizzle the basil-flavored oil over 6 small salad plates.
Let the tomatoes cool slightly. Place 2 tomatoes on each plate. Remove toothpicks and serve. Makes 6 servings.