Preheat oven to 175°C (347°F) with top and bottom heat.
Place beef brisket in a roasting pan and sprinkle with brown sugar and these spices: cinnamon pieces, mustard seeds, black peppercorns, cloves, allspice grains, juniper berries, crumbled bay leaves, and ginger powder.
Add onion quarters, carrot pieces, and celery to the pan around the brisket.
Pour water into the pan until the brisket is just covered.
Cover with a lid and bake for 2 hours 30 minutes.
Remove from oven. For Reuben sandwiches, cool to room temperature and slice. For a hot meal, rest for 10 minutes, then slice and serve with steamed white cabbage and mashed potatoes.