Trim brisket of excess gristle if needed. Do not remove the back-strap of fat as this will season in the cooking. In a 20 quart (or large enough to hold all the ingredients) pot place the brisket pretty side up. Add the spice packet that may have come with the brisket and also - beef broth, bay leaves, peppercorns, coriander seeds, whole allspice, dill seeds, mustard seeds, malt vinegar, whole cloves, packed brown sugar, apple juice, Guinness beer, sea salt & coarsely chopped garlic cloves. Now pour over just enough water to barely cover the brisket. Bring pot to a boil, cover and turn down temperature to simmer on medium for 2 hours. Meanwhile (you have 2 hours, take your time) wash/rinse and remove eyes from potatoes, quarter them, rinse again and set aside in bath of cool water. Also chop the sweet onion into large wedges too and set aside.
After 2 hours of simmering, drain and add the potatoes to the brisket pot. Also add 1/2 the onion and 5 baby carrots to the pot. Add more water if needed to cover everything; this may not be needed. Bring pot back to a boil, cover and turn down to simmer for another 30 minutes. Meanwhile, take the cabbage and peel off the outer leaves, rinse the heads and slice into large quarters. * Cut cabbage down the spine so the wedges stay together. Now add the cleaned cabbage, the remaining onion and the remaining 10 baby carrots to the pot. Cover and simmer again for another 30 minutes at least, stir once or twice. It is really hard to overcook cabbage so rather, just do not undercook it. After 30 minutes check to see if the cabbage is tender, if its not simmer for another 5-10 minutes. If cabbage is tender turn off the burner and let the pot cool a moment on the stove.
Remove the brisket from the pot, cut lengthwise and place on serving dish with the cabbage. Meanwhile in a large serving bowl, chop the stick of butter into smaller pats. Chop the parsley from its stems and add to the bowl with the butter. Strain out the hot potatoes/carrots/onions from the broth and put right into the bowl, stir until potato mixture is coated in parsley and butter is completely melted. You can toss out the broth or retain the last 1/4 (with the spices) for an au jus sauce.
Serve all with a dash of sea salt, mustard and horseradish if desired.