Place the olive oil and bacon fat in a large saucepan. Bring up to heat over medium. Once hot, place onions in the pot and simmer until slightly warmed. Generously add salt and pepper (about 1 - 2 tablespoon each) and stir until combined.
Add in the 2 tablespoons of butter to the onions, stir to mix. Add in the thyme and bay leaves. Keep the heat over medium-low and simmer until translucent. This will take awhile, upwards of an hour and a half depending on your stove.
While you're waiting for the onions to caramelize, slice up a loaf of sourdough bread and coat it generously with olive oil. Preheat the oven to 350 degrees and bake until browned (about 15 minutes). Flip half way through the cooking time. Set aside to cool.
Once the toast is cool enough to touch, take a peeled clove of garlic and gently rub it on each side of the toasts. Set aside.
Once the onions have reduced significantly, turned a deep brown color and left a brown residue on the bottom of the pan, take 2 tablespoons of flour to coat the onions. Then, add in 3 ounces of brandy and deglaze the pan. Let the onions absorb the brandy and cook until evaporated.
Add in 8 cups of beef broth. If you're not using homemade, or just want extra depth of flavour, add in 1 tablespoon both of worcestershire sauce and tomato paste. Simmer for another 10 - 15 minutes, until thickened and slightly reduced.