Polish Pizza (Zapiekanki)

Prep time: 15 minutes Cook time: 20-25 minutes Materials: 3 sub rolls opened 6 tbs butter (salted or unsalted are fine) 2 large onions cut into quarters 8 oz white mushrooms chopped 1 cup shredded mozzarella cheese 2 tbs vegetable oil Salt and pepper to taste Ketchup (optional) Process: 1. Preheat the oven to 400…