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.
I enjoyed this salad very much. So simple to just throw everything on the same pan. I removed the peppers and onions after 12 min and let the meatballs go 15 min just to be sure they were cooked through. I especially love the versatile turkey meatball recipe used here. It makes a big batch to keep in the freezer and they cook from frozen. I can fix my kids' favorite (spaghetti and meatballs) and my husband and I can have this salad if we feel like keeping it light by skipping the pasta.