In a Dutch oven, heat 2 tablespoons of olive oil and 1 tablespoon of butter over medium heat. Once melted, add 8 cups of thinly sliced onions and sauté for 10-13 minutes until softened and translucent.

Reduce heat to medium-low and continue cooking the onions for 30-40 minutes, stirring occasionally, until deep golden brown. Add 3 cloves of minced garlic and cook for another 2 minutes.

Stir in ½ cup of port wine or dry sherry, scraping the bottom of the pot. Bring to a boil, then simmer until reduced by half, about 5-10 minutes.

Pour in 64 ounces of beef broth, along with ½ teaspoon of pepper and ¼ teaspoon of salt. Bring back to a boil, cover, and simmer gently for 1 hour.

Preheat your oven to 400°F. Brush 24 slices of French baguette with remaining olive oil and toast for 3-5 minutes per side until golden and crispy. Rub with halved garlic cloves.

Ladle hot soup into broiler-safe bowls. Top with two toasted baguette slices and sprinkle with ¾ cup of shredded Gruyère or Swiss cheese. Broil until bubbly and golden brown, about 2-4 minutes.