Brush grill rack with vegetable oil. Heat gas or charcoal grill. In medium bowl, mix turkey, 3 tablespoons pasta sauce and chopped onion. Shape mixture into 2 patties, each about 3/4 inch thick and the approximate shape of the baguette pieces.
Cover and grill patties over medium heat 12 to 15 minutes, turning once, until thermometer inserted in center of patties reads 165°F. Top patties with cheese. Cover and grill about 1 minute longer or until cheese is melted.
Slice baguette pieces horizontally in half. Place lettuce leaves on bottom halves; top with burgers and onion slices. Top with remaining baguette halves. Serve with additional pasta sauce.