Preheat a grill to medium. Combine the beef, sausage, garlic, egg, breadcrumbs, parmesan, 3/4 teaspoon salt and a few grinds of pepper in a large bowl; mix with your hands. Form into twelve 1 1/2-inch meatballs.
Toss the bell peppers and scallions with the olive oil in another large bowl; season with 1/4 teaspoon salt and a few grinds of pepper. Oil the grill grates and arrange the meatballs, peppers and scallions on the grill. Cover and grill the vegetables, turning once, until lightly charred and tender, 2 to 3 minutes for the scallions, 5 to 7 minutes for the peppers. Grill the meatballs, turning occasionally, until browned and cooked through, 10 to 12 minutes. Toast the rolls cut-side down on the grill.
Cut the scallions into pieces. Spread 2 tablespoons ricotta on the bottom half of each roll. Top with the meatballs, scallions and peppers.