Portobello Mushrooms with Grilled Onions and Cheddar Cheese

Name: Portobello Mushrooms with Grilled Onions and Cheddar Cheese
Ingredients: 2 large portobello mushrooms, stemmed and wiped clean; Two ½-inch-thick slices red onion; 2 tablespoons olive oil; Salt and freshly ground black pepper to taste; 2 round Italian rolls; ¼ cup Chipotle Ketchup; 3 ounces cheddar cheese, thinly sliced