Rinse brisket; discard brine. Place brisket in a large pot. Add enough water to cover by 2 inches. Add onion, celery, and carrot; bring to a boil. Reduce heat, cover, and simmer until very tender, 3-3-½ hours.

Transfer beef to a cutting board. Tent with foil, and let rest 30 minutes.

Heat oven to 400º.

Discard remaining solids from broth and strain. Discard all but ½ cup broth.

In a large bowl, add cabbage and drizzle with olive oil to coat lightly. Arrange in a single layer on a foil-lined baking sheet and place in oven.

Add baby carrots and turnips to the bowl, drizzle with oil, and season with salt and pepper. Arrange in a single layer on a foil-lined baking sheet and place in oven.

Add Brussels sprouts to the bowl, drizzle with oil, and season to taste with salt and pepper. Arrange in a single layer on a foil-lined baking sheet and place in oven.

Add potatoes to the bowl, drizzle with oil, and season with salt and pepper. Arrange in a single layer on a foil-lined baking sheet and place in oven.

Turn vegetables every 15 minutes or so until well-browned. Remove to a platter once they’re cooked, and tent with foil.

Trim excess fat from beef. Slice thinly against the grain and transfer to a platter. Serve with broth, mustard, and roasted vegetables.