Dice the onion and slice the bell pepper. Heat 1 tablespoon of butter in a large skillet over medium heat. Add the onions, bell peppers, and mushrooms and cook until tender. Transfer cooked vegetables to a small bowl.
In your skillet, heat ½ tablespoon of butter. Place one tortilla into the pan, top with Gouda cheese and sauteed veggies, and top with another tortilla. Press on the top tortilla to allow the melted cheese to adhere to the top tortilla. Carefully flip the quesadilla over, and cook the other side for 1-2 minutes, or until golden brown. Repeat with remaining toppings and tortillas. Slice into wedges and serve with sour cream and salsa.