Preheat oven to 375°.
Line jelly roll pan with parchment paper. Spray lightly with cooking spray.
Place cauliflower in prepared baking pan (allow space between pieces). Brush lightly with olive oil, sprinkle with red pepper flakes and season to taste with fresh ground salt and pepper.
Roast in oven for 30 minutes or until tender. While cauliflower is roasting, prepare mushroom topping.
Begin by melting 1 Tbsp butter in a heavy skillet.
Add chopped onions and garlic and saute for 2 minutes. Stir occasionally to avoid burning.
Add mushrooms in a single layer on top of the onion mixture. Lightly saute until the edges begin to turn brown, turn over.
Stir in one extra Tbsp of butter and continue to turn mushrooms until preferred tenderness.
In a small bowl, mix swiss and mozzarella cheeses together, set aside.
Remove cauliflower from oven. Push cauliflower pieces together in center of pan.
Sprinkle with cheeses and carefully spread mushrooms evenly on top.
Return to oven for 5-8 minutes or until cheeses are melted.