Use a serrated knife to cut the top off of each tomato. Use a grapefruit knife (or a paring knife) to cut out the inside of the tomato. You may need to use your fingers to pull it out after cutting. Use a spoon to remove any leftover seeds or pulp.
In a blender or food processor, puree the spinach with the sour cream, Parmesan, salt and pepper. Gently spoon the mixture into each tomato and top with the gruyere. Bake until the cheese is melted and bubbly, 20 to 25 minutes.