Remove corned beef brisket from package, and place in large dutch oven-type pot. If the corned beef came with a seasoning packet of spices, empty that over the beef. Add the quartered onions, celery, garlic cloves, peppercorns, whole cloves and bay leaves. Next add the beer if you're using it, then add enough cold water to completely cover the meat.
Bring the pot to a boil, cover, reduce the heat to simmer, and cook up to 4 hours, or until the internal temperature reaches at least 160F degrees. During this process, turn the meat a couple of times, about every 1 ½ hours. Cook's note: Simmering the meat more than less will result in a very tender corned beef brisket, I have found over the years. Any time the meat has been less than tender, it's been because I rushed it. The boiling process will break down and render the fat to be very tender and almost disappear with time. You'll begin to notice this each time you turn the meat. At the first turning, the meat is almost stiff as a brick.
Make Irish Soda Bread while meat is cooking. Recipe below.
Also, stir desired amount of prepared horseradish into sour cream or yogurt and refrigerate until ready to serve.
After meat is tender and has cooked the appropriate time (or more), remove from water and place on baking sheet or roaster pan to rest, fat side up. Pre-heat oven to 350F degrees.
Spread fat-top with mustard, covering in a thin layer, using more than called for if your brisket is big. Sprinkle with fresh ground black pepper and garlic salt. Next, generously sprinkle brown sugar on top of mustard layer, being careful not to get sugar on the pan (unless you don't mind your kitchen filling with a burnt sugar smell and some smoke). Let meat rest like this while you prepare the rest of the vegetables:
Bring the water back to a boil and add the red potatoes and carrots. Cover, reduce heat to medium and simmer until tender.
Place meat in oven for approximately 15-20 minutes, checking periodically. A nice carmelized crust will begin to form on top of the meat. If the meat begins to burn, tent with foil for last few minutes. While the meat is finishing in the oven, do the following:
Once the potatoes and carrots are tender, add the if you're making the boiled cabbage method, add cabbage and parsley to the water, continue to simmer, covered an additional 10 minutes or so, until the cabbage is tender. (If you are going to saute' your cabbage, my favorite method, just add the parsley here and see note below for sauteed cabbage instructions.)
Turn your vegetables off and:
Place meat on a serving platter or cutting board. Slice thinly, about ¼ inch thick.
Using a slotted spoon, remove vegetables into a serving bowl.
Serve meat with horseradish-sour cream mixture, vegetables, Irish Soda Bread and creamy soft butter, if desired.
Leftover meat is amazing the next day.
Following is the sauteed cabbage recipe, (BTW, this can be made ahead and reheated):
Heat 2 Tablespoons olive oil on medium high to high heat in a large, wide pot or large, high-sided sauté pan. Add chopped onions, cook for a couple of minutes, then add garlic.
Add a third of the sliced cabbage to the pan. Sprinkle with a little salt and stir to coat with oil and mix with onions. Spread out the cabbage evenly over the bottom of the pan and do not stir until it starts to brown. If the heat is high enough, this should happen quickly. The trick is to have the burner hot enough to easily brown the cabbage, but not so hot that it easily burns. When the bottom of the cabbage is nicely browned, use a metal spatula to lift it up and flip it, scraping the browned bits as you go.
Once the cabbage in the pan has browned on a couple of flips, add another third of the cabbage to the pan. Mix well, then spread out the cabbage and repeat. Add a bit of butter to the pan for flavor, and to keep the cabbage from sticking too much to the pan. Once this batch has cooked down a bit and browned, add the remaining third of the cabbage and repeat.