French Onion Soup In The Slow Cooker

French Onion Soup In The Slow CookerBy thekitchn.comThis recipe is very delicious, you will love it!Yield: 6 ServingsIngredients:3 pounds yellow onions, peeled, sliced, and cut into quarter-moons2 tablespoons unsalted butter, melted2 tablespoons olive oil2 teaspoons kosher salt, plus more as neededFreshly ground black pepper10 cups reduced-sodium beef broth2 tablespoons balsamic vinegar3