Spread the meatballs out in a 9x13" baking dish. Pour the soup out over the meatballs. Stir a bit to evenly coat the meatballs.
Bake the meatballs at 350 degrees for 20-25 minutes.
In a medium sized skillet, over medium heat, melt 2 tablespoons of butter. Stir in the onions, and then cook, stirring often to prevent burning, until the onions are caramelized- about 7-10 minutes. Remove from heat, and set aside.
Slice the baguettes in half, if they aren't already, and spread the rest of the butter evenly out over the insides. Transfer them to a baking sheet, butter sides up.
When the meatballs are done, turn the oven to broil. Place the bread in the oven, watching closely, and cook just until the butters melted and they've begun turning golden brown.
Using a slotted spoon, add meatballs to the bottom of each baguette. Top each generously with the caramelized onions and cheese. Place them back under the broiler, just until the cheese is melted & bubbly.
Remove them from the oven. Top each sandwich with its top bun, and serve immediately.