• In a large pot, bring together onions, smashed garlic cloves, beef stock, black peppercorns and Worcestershire sauce.  Cook over high heat and bring to a boil.
• Once at a boil, add dark beer/Guinness and stir.  Let bubble away for 2-3 minutes.
• Taste and add season with salt.  The amount of salt you put in will depend on how salty your beef stock is.  Start with 1/2 tsp and move on from there.
• Reduce heat to low and add brisket.  Fully submerge the brisket into the stock.
• You want the bare minimum of a simmer going for this so adjust heat accordingly or transfer to a slow cooker and cook on low heat.
• Cook low and slow for 5-6 hours.  This will depend on how big your brisket is.  Check on it from time to time and add water if needed.
• When tender to the touch, it is done.
• Slice into strips if you are having it as a main or slice thinly and dunk in gravy for use in sandwiches.

Gravy

• Using a sieve, strain away the debris of onions, garlic, peppercorns, etc away from the remaining stock. Toss debris.
• Take one cup of stock and bring to a boil on high heat in a small saucepan.
• Reduce heat to a low simmer then add Worcestershire sauce and yellow mustard and stir.
• Let bubble away for a minute or two then add 1 tsp of cornstarch mixed with 1 tbsp of water.
• Stir in cornstarch mixture and let thicken.
• Season with salt and pepper to taste.
• Drizzle over sliced brisket for extra deliciousness.