2 Tbsp olive oil, plus more to brush toasts
2 Tbsp unsalted butter
3 lbs yellow onions, about 6 large onions, halved, peeled and thinly sliced with the grain
1/2 tsp granulated sugar
2 garlic cloves, minced
1/2 cup dry sherry wine, or use dry vermouth or dry white wine
8 cups beef stock, or broth
1 bay leaf
3 sprigs fresh thyme, plus more to garnish (or 1/4 tsp dried thyme)
1 1/2 tsp salt, or to taste, divided
1/2 tsp black pepper, or to taste
12 slices baguette
8 oz gruyere cheese, 1 1/2 cups shredded, divided
Cut onions in half, cut off the ends, then peel and slice onions thinly (cut parallel with the grain of the onion).
In a large heavy-bottomed pot or dutch oven over medium heat, add 2 Tbsp olive oil and 2 Tbsp butter. Add sliced onions and sauté uncovered, stirring occasionally for 10 minutes.
Sprinkle the onions with 1/2 tsp sugar which will help the onions caramelize faster. Sauté uncovered for another 30-40 minutes, stirring occasionally until onions are caramelized and browned. Stir more frequently towards the end to prevent scorching or burning the onions.
Once onions are caramelized, add minced garlic and sauté another minute.
Add 1/2 cup sherry and deglaze the pot by scraping the bottom. Continue stirring until all of the sherry has cooked out (about 3 minutes over medium heat).
Add 8 cups beef stock, 1 bay leaf, thyme and 1 tsp salt. Partially cover and simmer for 30 minutes for the flavors to meld. Add another 1/2 tsp salt and 1/2 tsp black pepper, or season to taste then remove from heat.