Marinating the Beef:

In a large glass or ceramic bowl, combine Burgundy wine, brandy, onions, carrots, garlic, peppercorns, salt, parsley, and bay leaf. Toss the beef in the marinade, ensuring it is evenly coated. Cover the bowl with plastic wrap and place it in the refrigerator for 48 hours to allow the flavors to develop fully.

Preparing the Beef Bourguignon:

Preheat your oven to 300°F (150°C). Drain the beef from the marinade and pat dry with paper towels. Strain the vegetables, reserving the marinade for later use. Heat 2 tablespoons of olive oil in a large skillet over medium-high heat. Brown the beef in batches on all sides, then transfer to a bowl.

Cooking the Bacon and Deglazing the Pan:

In the same skillet, cook the diced bacon until lightly browned. Add the bacon to the bowl with the beef. Drain any excess fat from the pan and pour in 1 cup of the reserved marinade. Bring it to a boil while scraping up the browned bits from the bottom of the pan with a wooden spoon. Return this liquid to the remaining marinade.

Sautéing the Vegetables:

Heat the remaining 2 tablespoons of olive oil in the skillet. Add the chopped onions and sauté until tender, about 2 to 3 minutes. Stir in the reserved carrots and onions from the marinade and cook briefly. Transfer these vegetables to the bowl with the beef.

Thickening the Sauce:

Return the skillet to the heat and stir in the flour, cooking it in the pan drippings until browned, around 1 to 2 minutes. Stir in the crushed garlic and tomato paste, mixing thoroughly. Pour in the beef broth and the remaining marinade, whisking until smooth to avoid lumps. Season with salt and pepper. Bring the mixture to a boil, then pour it over the beef and vegetables in the bowl.

Baking the Bourguignon:

Transfer the beef mixture to a casserole dish or Dutch oven. Bake in the preheated oven for approximately 3 hours, stirring occasionally. If the liquid reduces too much during cooking, add water as needed to maintain moisture.

Sautéing the Mushrooms:

About 10 minutes before the bourguignon is ready to serve, melt butter in a skillet over medium-high heat. Sauté the sliced mushrooms until golden brown, about 5 to 7 minutes. Stir the mushrooms into the beef mixture, combining thoroughly.