Scale

1/2 cup unsalted butter (See Notes 1 for substitutions)

4 medium white onions, sliced

2 medium garlic cloves, finely chopped (See Notes 2 for substitutions)

2 bay leaves

2 fresh thyme sprigs, leaves picked

3 tablespoons all-purpose flour (See Notes 3 for substitutions)

9 cups beef bone broth (See Notes 4 for substitutions)

1 teaspoon beef bouillon paste (See Notes 5 for substitutions)

Freshly ground black pepper, to taste

Mozzarella Croutons, for topping

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In a Dutch oven or large stockpot, melt the butter over medium heat.
Add onions, garlic, bay leaves, and thyme leaves. Sprinkle in about 1 teaspoon salt. Stir occasionally and cook until onions are very soft and golden brown—about 30–40 minutes.
Stir in 1 cup of beef broth, the bouillon paste, and Worcestershire sauce. Bring to a boil, then reduce heat and simmer until liquid has evaporated and onions look “dry,” about 5–10 minutes.
Discard bay leaves. Sprinkle flour over the onions and stir. Lower heat to medium-low and cook 10 minutes to remove the raw flour taste.
Add remaining beef bone broth. Bring soup to a gentle simmer and cook another 10 minutes. Season with salt and pepper to taste.
Optional: Blend the soup for a smoother texture (this also lightens the color).
Ladle into bowls or mugs. Top with cheesy mozzarella croutons and serve hot.

Butter Substitute: Use extra virgin olive oil, sunflower oil, or vegetable oil (1:1).
Garlic Substitute: Use ½ teaspoon garlic powder if fresh garlic isn’t available.
Flour Substitute: Use cornstarch, arrowroot, or tapioca starch (1:1) for gluten-free.
Beef Bone Broth Substitute: Regular beef stock works just as well (1:1).
Bouillon Paste Substitute: 1 beef bouillon cube = 1 teaspoon paste.