Preheat oven to 350 degrees. Butter one side of 4 slices of bread. Spread 1-2 tablespoons of softened cream cheese on each of 2 slices of bread. Top with slices of Pepper Jack and Cheddar cheese. Add heirloom tomatoes and spinach leaves to the top of the cheese. Finish sandwiches with slice of bread, buttered side up.
Place sandwiches on baking sheet and bake for 20-25 minutes, or until cheese has melted and bread is lightly toasted, flipping halfway through.