Dice the bacon and cook in a Dutch oven or ovenproof stockpot until brown and crisp. Remove and set aside.

Add olive oil to the remaining fat in the Dutch oven and heat over medium-high heat.

Pat the beef dry with paper towels and brown in batches, without overcrowding, removing when done. Set aside.

Brown the sliced onions in the remaining fat, remove, and set aside. Discard any remaining fat.

Reduce heat to low, add the cognac, and scrape up all the browned bits. Carefully ignite with a match and swirl to burn off the alcohol.

Preheat oven to 325°F (160°C).

Return the bacon, beef, and onions to the Dutch oven.

Stir in salt, thyme, pepper, bay leaves, garlic, brown sugar, beer, and broth.

Spread mustard on the bread slices and lay them on top of the stew.

Bring to a boil, then remove from heat, cover, and place in the oven.

Cook for 2 to 2.5 hours, or until the meat is fork-tender, stirring occasionally.

Before serving, skim the fat from the surface of the sauce.

Stir in red wine vinegar before serving.

Refrigerate leftovers for up to 2 days or freeze for up to 3 months. Reheat gently over medium-low heat.