Trim away any excess fat from the brisket. Rub it generously with salt and pepper. Brown the brisket on both sides in a heavy roasting pan over high heat for about 5 minutes. Remove the brisket and pour off all the fat. Place the onions, celery, and carrots in the pan, reduce the heat to medium, and scrape the bottom, adding liquid if needed to deglaze pan. Add the parsley stems, red pepper, thyme, garlic, bay leaf, beer, stock, tomato paste, salt, and pepper. Bring to a boil, return the meat to the roasting pan, cover, and bake for 2-1/2 to 3 hours, or until tender. Skim off the fat.