Heat butter in large deep-sided ovenproof skillet or Dutch oven over medium-low heat. Add onion and garlic and cook until onion is translucent.

Add mushrooms and peppers to skillet. Cook until mushrooms release their liquid and peppers are crisp-tender.

Preheat oven to 350°.

Add beef to skillet and mix with veggies. Top with provolone and sprinkle on mozzarella.

Bake 15 min, then finish under broiler 2-3 min until cheese is browned and bubbly.