Family dinners cooked in a Crock Pot can be so delicious and full of flavor. This homemade French Onion Soup can easily become your staple for family meals.
Season the onions. Place the onions in a 5-quart or larger slow cooker. Stir in the butter, oil, salt, and a generous amount of pepper.
Cook on LOW for 6-8 hours. Cover and cook on the LOW setting overnight until the onions become dark golden-brown and soft.
Stir in the broth and balsamic vinegar.
Cook on LOW for extra 6 hours. Cover and continue cooking on the LOW setting for about 6. This is flexible. As long as your slow cooker holds moisture well, you can cook the soup for hours. Longer cooking will only intensify the flavors. Taste and season with more salt and pepper if needed.
Portion the soup into oven-safe bowls. Arrange a rack in the upper third of the oven and heat to 350°F. Ladle the soup and onions into oven-safe soup bowls.
Top with toast slices (store-bought or homemade, see my post) and shredded cheese. Top each bowl with a slice or two or three of toast and a generous quantity of shredded Gruyère cheese, about 1/3 cup per bowl (depends on how big the bowl is).
Place the bowls on a rimmed baking sheet into the oven for about 30 minutes.
When the cheese is melted and looks a bit toasty, remove the soup bowls from the oven and top with fresh green onions and herbs if you wish.