Preheat oven to 350 degrees F. In a large skillet over medium high heat, cook beef and onion until beef is no longer pink, 8-10 minutes, crumbling the beef as it cooks; drain well; season to taste with kosher salt. To prepare the cheeseburger filling, stir in the tomato sauce, ketchup, mustard and black pepper until nicely blended; stir in 3/4 cup shredded cheddar cheese. Spoon approximately 2 tablespoons of the prepared cheeseburger filling into each cooked pasta shell, placing each one…