Preheat oven to 400 °F.  Place a large baking sheet in the oven to preheat for about 10 minutes.

Heat garlic with cooking spray in a medium sauté pan over medium heat.  Cook until soft, but not browned.  Add onions, and continue to cook until the onions are soft, but not browned.

Add spinach, and toss gently.  Cook until the spinach is heated throughout.  Season with pepper, and set aside to cool.

When the spinach and onions are cool, assemble each sandwich with one slice of bread on the bottom, one tomato slice, ½ cup of spinach mixture, ¼ cup of cheese, and a second slice of bread on the top. (For picky eaters, see Healthy Eating Two Ways suggestion below.)

Spray the preheated nonstick baking sheet with cooking spray.  Place the sandwiches on the baking sheet.  Bake for 10 minutes, or until the bottom of each sandwich is browned.

Carefully flip sandwiches, and bake for an additional 5 minutes, or until both sides are browned.  Serve immediately.

TIP: For picky eaters, start with less spinach in the sandwich, and possibly serve the remaining amount on the side.

source: National Heart Lung and Blood Institute