Season the beef chuck cubes with salt and pepper, then heat vegetable oil in a Dutch oven over high heat until shimmering. Sear the beef until deeply browned, turning as needed, then remove to a plate.

Add the sliced onions to the Dutch oven and cook over medium heat for 20-25 minutes, stirring occasionally, until very soft and golden. Add the minced garlic and cook for 1 minute, stirring, until fragrant.

Sprinkle in the all-purpose flour, whisk it in, and cook for 1 minute to remove the raw flour taste. Pour in the dark Belgian ale and deglaze, scraping up all browned bits from the bottom.

Whisk in the beef broth, tomato paste, red wine vinegar, brown sugar, Dijon mustard, thyme, and bay leaves, then add the browned beef back to the pot. Bring to a boil over high heat, then reduce heat, cover, and braise for 2 hours until the beef is very tender.

Remove the bay leaves and thyme, taste, and season with additional salt and pepper if needed. Serve hot with crusty bread alongside.