Spread 1/2 teaspoon butter onto 1 side of each bread slice. Lay 1 bread slice, butter-side down, in the hot waffle iron. Spread 1/2 teaspoon mustard onto bread slice in waffle iron; top with 1/4 cup spinach, 2 slices mozzarella cheese, 1 teaspoon onion, half the tomato slices, and 1/4 cup spinach, respectively. Lay 1 slice bread, butter-side up, atop the spinach layer.