Pat the beef dry. Season generously with salt and pepper.
Heat the oil in a heavy Dutch oven over medium-high heat. Brown the beef in three batches, 5 minutes per side. Set aside.
Brown the mushrooms hard for 6 minutes in the same pot. Set aside with the beef.
Add the pearl onions, brown for 5 minutes. Add the garlic and tomato paste, cook 1 minute.
Sprinkle in the flour, stir for 1 minute. Pour in the ale, scrape the pot, reduce by half (about 8 minutes).
Return the beef and mushrooms. Add stock, brown sugar, Worcestershire, mustard, thyme, bay.
Cover, simmer over the lowest heat for 2.5 to 3 hours, until the beef collapses to a fork.
Cool completely. Refrigerate overnight if possible.