In large skillet, melt butter over medium heat. Add mushrooms, onions, garlic and cayenne; salt and pepper to taste. Sauté until mushrooms are softened (about 3-5 minutes). Add spinach (in batches, if needed); continue sautéing until spinach has completely wilted.
Stir in flour; cook 1 minute. Stir in half & half and mustard, cook, stirring constantly, until thickened. Add ½ cup Gruyere and ½ cup Parmesan; salt and pepper to taste. Mix until well combined.
Transfer to a 2-quart baking dish that has been sprayed with cooking spray. Sprinkle with remaining cheeses. Bake in a preheated 375°F oven 30 minutes or until golden brown and bubbly. Let stand 5 minutes before serving.