Melt 2 tbsps butter in medium-size sauté pan over medium heat.

Sauté onions until brown and caramelized, about 15–20 minutes.

Set aside.

Toss Swiss & Gruyere the cheese with flour in mixing bowl until evenly coated.

Set aside.

Combine beef broth, cooking sherry, and concentrated beef broth in a medium sauce pot over medium-low heat and bring to a slight simmer.

Whisk the cheese into the liquid in four or five batches. Wait for the cheese to melt before adding next batch of cheese. Don't let the mixture come to a boil.

When final batch has melted completely, remove pot from heat and stir in caramelized onions and garlic pepper.

Transfer mixture to a serving bowl or fondue pot

Serve with French bread (or whatever else you want to use.. potatoes, cauliflower, asparagus...)