Sprinkle steaks lightly with salt and black pepper. Cut a 2-inch-wide pocket in the side of each steak, cutting to but not through the other side. Cut six 2x1-inch pieces roasted red pepper; set remaining peppers aside. Place a red pepper piece and one or two basil leaves in the pocket of each steak.
Brush 3 tablespoons of the vinaigrette on steak and squash. Grill steaks on the rack of an uncovered grill directly over medium heat for 7 minutes. Turn steaks and add squash. Grill for 7 to 9 minutes more for medium doneness, turning squash once.
Meanwhile, for relish, in a food processor combine remaining sweet peppers, garlic, green onion, and herb. Process until finely chopped. Stir in remaining vinaigrette. Arrange steak and squash on a platter. Serve with relish. Makes 6 servings.