This is my favorite Onion Soup Recipe. Made by slowly caramelizing six different types of onions, it’s comforting, fragrant, flavorful, and especially delicious when topped with baked Parmesan crisps or gooey melted cheese.
Place 2 tablespoons of olive oil and 2 tablespoons of butter in a large heavy-bottomed Dutch Oven or stockpot set over medium-high heat. Add the leeks and cook, stirring often, until starting to soften (approximately 5-6 minutes). Add the sliced shallots to the leeks and mix well to combine. Continue to cook, stirring frequently, for 3-4 minutes. Add the sliced green onions, white onions, red onions, yellow onions, salt, and black pepper.
If you find that all the onions refuse to fit into the pot, that's ok! You may need to add part of the onions, allow them to cook down, add some more onions, allow those to cook down, and then add the rest.
Drizzle the remaining olive oil over the onions and add the remaining 2 tablespoons of butter to the pot. Cook the onions, stirring frequently until softened, deeply golden-brown, and jammy (about 60-90 minutes). In the last 15 minutes, stir in the garlic, fresh parsley, rosemary, and thyme. Mix well.
With the onions deeply browned and caramelized, increase the temperature to high and add the dry sherry (or cognac) and scrape any burnt pieces stuck to the bottom of the pot. Allow the alcohol to cook for approximately 1-2 minutes before adding the beef broth to the pot. Cover and bring to a boil, then reduce the heat to low. Simmer for an additional 20-30 minutes and season with additional salt and pepper, as needed.
Serve each bowl topped with homemade baked Parmesan crisps (see below), fresh chopped parsley and additional shredded cheese, if desired.
For the Baked Parmesan Crisps
Prepare the Parmesan crisps approximately 15 minutes before the soup is ready to be served. Position the baking rack in the upper third of the oven and preheat the broiler to low.
Slice the baguette into thin 1/2-inch slices and transfer to a baking sheet in a single layer. Brush the top side of each slice with oil. Transfer baking sheet to the oven and broil until golden brown, approximately 4 minutes.
Remove the baking sheet from the oven and immediately flip over each slice and top with approximately 1 tablespoon of shredded Parmesan cheese. Return the baking sheet to the oven and broil an additional 3-4 minutes or until the cheese has melted. Serve.