In a large saucepan over medium-high heat, melt butter and add oil. Add onions, garlic and sea salt; sauté for 2 minutes. Add pepper, stir, reduce heat to medium and cover; cook for 8 minutes, stirring every couple minutes. Remove lid, reduce heat to medium-low and continue cooking for another 10 minutes, stirring often.
Pour Marsala wine and continue stirring; cook until wine has almost evaporated. Pour beef broth and vegetable broth; bring to a simmer. Add thyme, garlic salt and bay leaf; cook for 15 minutes. When ready to serve, remove thyme and bay leaf.
Preheat broiler and place rack on second row from the top. Place bread (cut slices to fit your bowl) on a baking sheet and toast. Remove from the heat, flip over and return to oven. When almost done, sprinkle cheese on two slices. Return to oven and cook until melted.
Place each toast with cheese at the bottom of the soup bowl. Ladle beef broth over and then place toast on top. Place two slices per bowl, sprinkle the remaining cheese over and transfer bowls under the broiler. Cook until cheese is melted and browned, about 3-4 minutes.
Note: For a non-alcohol version, use sherry vinegar.