Preheat the grill to medium-high. Trim the tops and removed the seeds and membranes from the bell peppers. Cut them into wedges and place in a bowl. Toss in 2 Tbsp. of the oil, salt and pepper. Grill until nicely coloured on both sides; arrange on a platter. Drizzle with remaining olive oil and balsamic vinegar; sprinkle with basil, onion and cheese. Let stand a few minutes to allow flavours to meld and then serve.