1. Preheat oven to 475°F. Line a rimmed baking sheet with foil. Arrange peppers and onions on baking sheet. Drizzle with oil and season with salt and pepper. Toss to coat. Add sausages to the sheet.
2. Roast until the vegetables are tender, about 18 minutes.
3. Heat the broiler with the rack in the top oven position. Add cheese slices to the top of the vegetables. Broil until the cheese is melted and the sausages are cooked through, about 3 minutes. Remove from the oven.
4. Spoon pesto over and drizzle balsamic vinegar. Garnish with torn basil leaves.
5. Serve with crusty bread if desired.