1 small red onion, finely sliced
2 cups cherry tomatoes, split in half
1 medium red bell pepper, cut into 3/4-inch pieces
1 medium green bell pepper, cut into 3/4-inch pieces
Kosher salt and freshly ground black pepper
1 tablespoon balsamic or red wine vinegar
1 teapoon dijon mustard
3 tablespoons extra-virgin olive oil
4 ounces feta cheese, cut into 1/2-inch cubes
1/2 cup chopped fresh basil leaves