Brown and fully cook Italian sausage. Drain fat and set aside.
Position a rack in the top shelf of the oven and preheat to broil. Arrange foccacia on a large, heatproof platter in a single layer to overlap slightly. Sprinkle the Italian sausage, red onion, cubanelle pepper and cheese over foccacia.
Broil until the cheese is bubbling, 3 minutes. Remove the platter from the oven, top with balsamic vinegar and sliced fresh basil.