Slow Cooker French Onion Soup

• 1 Shallot or fresh onion

• 3 lbs Yellow onions

• 10 cups Beef broth, reduced-sodium

• 2 tbsp Balsamic vinegar

• 1 Black pepper, Freshly ground

• 2 tsp Kosher salt

• 2 tbsp Olive oil

• 4 Baguette slices per bowl, toasted

• 2 tbsp Butter, unsalted

• 1/3 cup Gruyere cheese per bowl

• 3 tbsp Brandy