Chop the stems from 12 medium baby Bella mushrooms. Place mushroom caps on a baking sheet sprayed with nonstick spray, rounded sides down. Bake at 375 degrees until tender, 12 - 14 minutes.
Meanwhile, bring a skillet sprayed with nonstick spray to medium heat. Cook and stir chopped stems, 1/4 cup diced onion, and 1/4 tsp. chopped garlic until slightly soft, about 3 minutes.
Pu the mixture into a medium bowl, and pat dry. Add a 2.6-oz albacore tuna in water, a wedge of The Laughing Cow Light Creamy Swiss cheese, 1 tbsp. Dijonnaise, and 1 tbsp. sweet relish (patted dry). Stir until thoroughly mixed.
Blot moisture from mushroom caps, and fill with tuna mixture. Bake until filling is hot, about 8 minutes.
Top each mushroom with a quarter slice of 2% milk American cheese. (That's 3 whole slices in total.) Bake until melted, about 1 minute. (3 mushrooms: 94 calories, 3g fat)