Preheat oven to 400°F. In a large oven-safe skillet or Dutch oven, cook ground beef and onion over medium-high heat until the meat is no longer pink, about 5-7 minutes. Drain off the excess grease.
Stir in the baked beans, cannellini beans, barbecue sauce, maple syrup, and mustard. Season with salt and pepper to taste. Bring the mixture to a boil.
Once the mixture boils, transfer the skillet or Dutch oven to the oven. Bake, uncovered, for 10-15 minutes, stirring halfway through, until bubbly.
Serve with cornbread.
