Heat oil in a large nonstick skillet over medium - high heat. Add mushrooms to pan. Sauté 4 minutes or until tender; flipping once.
Add garlic to pan; sauté for 30 seconds. Season mushrooms with salt and pepper, then set aside.
In a small mixing bowl, combine Gorgonzola and mayonnaise. Stir well. Spread about 1/4 the mixture onto each hamburger bun. Top each hamburger bun with 1 mushroom, one half cup spinach and 2 Tbsp. red peppers.