Heat broiler, with rack set 4 inches from heat. On a rimmed baking sheet, toss scallions and peppers with 2 teaspoons oil; season with salt and pepper and push to one side of sheet. Place meatballs on other side of sheet; toss with 2 teaspoons oil.
Broil, tossing vegetables and turning meatballs occasionally, until meatballs are cooked through and vegetables are lightly charred, 10 to 12 minutes. Remove from oven, and cool for 3 minutes.
Meanwhile, in a large bowl, whisk vinegar and mustard with remaining 2 tablespoons oil; season with salt and pepper. Add lettuce, scallions, and peppers; toss to combine. Top with meatballs and Parmesan.