Melt the butter in a large heavy bottom pot, sweat the onions until they are golden brown, about 10 minutes. Add beef broth, Kitchen Bouquet, sachet bag, and port. Bring to a boil and let simmer for 40 to 45 minutes. Taste and adjust seasonings with salt and freshly ground black pepper. Remove sachet before serving.
To assemble; pour soup into oven proof bowl, top with crouton and cheese, bake in a pre-heated 400°F oven until the cheese starts to melt and brown on the side of the bowl, about 8 to 10 minutes.
To make homemade croutons, use 6 slices of bread: trim to fit the opening of the bowl, brush with butter and cook in oven to dry (400°F) for 3 to 5 minutes; turn bread and repeat the process.