Melt Butter in a large pot (I like using my Le Creuset 7 1/4 quart pan), add sliced onions, garlic, rosemary, salt, and pepper and cook until onions are very soft and caramelized (about 30 mins).
Add wine and bring to a boil, reduce heat and simmer until wine evaporates and onions are dry – about 5-10 minutes. Discard rosemary. Dust onions with flour and give them a stir.
Turn the heat down to medium so the flour doesn’t burn and cook for 10 minutes to cook out the flour taste.
Add your beef broth, and reserved pieces of slab bacon (leftover from broth).
Simmer for about 30 minutes, and season with salt and pepper until it is absolutely AMAZING.
Preheat oven to 350 and slice the baguette into thin slices. Rub olive oil and a garlic clove on baguette slices and sprinkle with grated gruyere.
Prepare soup ramekins on a baking sheet and ladle soup into ramekins. Add a baguette slice with gruyere to the top of each ramekin and sprinkle more gruyere over the top.
Bake for 2-5 minutes until the top of the cheese is golden brown.