Apply butter one side of each slice of bread,  place one slice of cheese on each inside of both slices. Add tomatoes on top of the cheese on one slice and then spinach. Sprinkle some salt and pepper to taste and sit the other slice on top with butter side up. Place butter side down on a stovetop or griddle over medium heat. Grill sandwiches until golden brown, about 2 to 3 minutes each side. Serve hot.