1. Preheat oven or grill to 400°F Spread the butter on each slice of bread and toast on baking sheets in the oven, or grill, until golden brown, about 5 minutes per side.
2. In an omelet pan or small sauté pan, heat the oil over medium heat until it shimmers. Sauté the garlic until it turns light brown and then add the tomato and spinach. Season with salt and pepper. Cook spinach for 1 to 2 minutes, until wilted a little but not totally limp. Add the eggs and cook until they are solid enough to flip, about 2 to 3 minutes. Once flipped, add the cheese and cook for 30 seconds to 1 minute more. Place omelet on toasted bread. Slice and top with other slice.
TIP: This dish can also be served as a brunch frittata.