How to Make It
Cook the bacon in a Dutch oven or ovenproof stockpot until brown and crisp. Remove with a slotted spoon and set aside.
Add the olive oil to the fat remaining in the Dutch oven. Heat over medium-high heat. Pat the meat dry with paper towels and brown very well, a few pieces at a time (don't crowd the Dutch oven), removing them when done. Brown the 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 light with a match and swirl to burn off the alcohol.
Heat oven to 325° F. Return the bacon, beef, and onions to the Dutch oven. Stir in the salt, thyme, pepper, bay leaves, garlic, brown sugar, beer, and broth. Spread the bread with the mustard and lay on top. Heat to boiling. Remove from heat, cover, and place in the oven. Cook 2 to 2 1/2 hours or until the meat is fork-tender, stirring occasionally. Before serving, skim the fat from the sauce. Stir in the vinegar.
KEEP IT: Refrigerate any leftovers for up to 2 days. To freeze, place in airtight containers or heavy-duty zipper bags up to 3 months. When reheating, warm gently over medium-low heat.