Season and Sear: Pat the beef cubes dry with paper towels. Season generously with salt and pepper. Heat oil in a large Dutch oven or heavy-bottomed pot over medium-high heat. Working in batches to avoid crowding, sear the beef on all sides until deeply browned (about 3-4 minutes per batch). Remove beef and set aside on a plate.
Caramelize Onions: Reduce heat to medium. Add sliced onions to the same pot. Cook, stirring occasionally, for 8-10 minutes until softened and golden brown. Stir in minced garlic and cook for 1 minute until fragrant.
Build the Base: Stir in tomato paste and cook for 1 minute to deepen its flavor. Pour in the beer and beef broth, scraping up any browned bits from the bottom of the pot with a wooden spoon. Add Worcestershire sauce and thyme sprigs. Bring to a gentle simmer.
Braise: Return the seared beef and any accumulated juices to the pot. The liquid should come about halfway up the meat, not fully submerged. If needed, add a splash more broth or water. Cover with a tight-fitting lid. Reduce heat to low and simmer gently for 2 hours.
Check Tenderness: After 2 hours, check for doneness. The beef should be fork-tender; if it still feels chewy, continue cooking in 15-minute increments until tender. Remove thyme sprigs.
Thicken the Sauce: In a small bowl, whisk flour and cold water to create a smooth slurry. Stir this into the simmering stew. Simmer uncovered for 5-10 minutes until the sauce thickens to your desired consistency. Taste and adjust salt and pepper as needed. Serve hot.
