Put the corned beef brisket in a large Dutch oven or stockpot and cover with water. Tie spices in a double thickness of cheesecloth or put them in a small bouquet garni bag. Add the spice bag to the water.
Place the pot over high heat and bring to a boil; reduce heat to low, cover, and simmer for 2 1/2 hours.
Add the potatoes, carrots, turnips, and peeled onions. Cover and simmer for 10 minutes longer.
Add the cabbage wedges, cover, and simmer for about 15 minutes longer, or until vegetables are tender. (Cabbage can be cooked in a separate pan if you don't have room, in a small amount of salted boiling water.) Cover the pan and simmer for about 15 minutes. Continue to simmer the potatoes and other vegetables until tender.
Heat the oven to 375 F/190 C/Gas 5.
Transfer the corned beef brisket to a baking pan.
Combine glaze ingredients in a saucepan. Bring to a simmer and cook, stirring, for 5 minutes. Spoon over corned beef
Bake in the preheated oven for about 10 minutes, basting a few times.
Transfer the glazed corned beef to a large serving platter and slice the meat. Arrange the cabbage wedges, potatoes, carrots, onions, and turnips around the sliced brisket.