2 medium yellow or sweet onions
3 garlic cloves, chopped
3 tablespoons flour
2 quarts beef stock
1 tablespoon olive oil
1 1/2 cups chicken stock
1/2 cup Marsala wine
1/4 cup of apple cider vinegar
2 springs fresh thyme
2 bay leaves
Salt and pepper, to taste
Slice onions into half-moons. Add oil and onions to a large stock pot; stir to combine. Cover and simmer for 15 minutes. Uncover the pot and cook until the onions begin to caramelize, about 15 minutes. Add garlic and flour and stir to combine. Let cook for 2 minutes. Add stock, wine, vinegar, thyme, bay leaves, and salt and pepper to taste. Stir. Simmer for 30 minutes to let flavors incorporate.
Slice and toast baguette slices.
Ladle soup into oven-safe bowls, place toast on top of soup, cover with Gruyere cheese slices, and place bowls onto a cookie sheet and under broiler to melt cheese.