Preheat oven to 425°F. Place bell peppers and onion in a large bowl. Add oil, salt and pepper; toss to coat.
Spread the vegetables in a single layer on a large rimmed baking sheet. Bake until tender and charred in spots, 20 to 25 minutes.
Transfer the vegetables to a large serving bowl. Toss with lemon juice, parsley and basil. Serve immediately.
