Start by cutting your heirloom tomato and avocado into thin slices. Set aside.
Heat up a nonstick skillet over medium heat. Rub butter directly on the skillet and add both slices of bread to the pan. Toast for a couple of minutes until golden brown, then remove the bread slices. Keeping in mind that the sides we just browned go on the inside, build your sandwich by first doing the sliced gruyere cheese, then the tomato, then the avocado. Season with salt and pepper. Add another layer of cheese, and add your other slice of bread. Add more butter to your skillet and brown the sandwich on each side for about 3 minutes, until both sides are golden brown and the cheese has melted. Enjoy!