Spray 9”x13” pan with non-stick cooking spray. Pre-heat oven to 350°. Cut onions into 1/4”-1/2” thick slices and separate into rings. Lay rings in dish. Sprinkle with garlic salt and pepper. Cut butter into pats and place on onions. Sprinkle cheddar and mozzarella over the onions. Top with parmesan.

Bake at 350° for 30 minutes until bubbly. Remove from oven and garnish with fresh oregano. Serve warm.