Start by preheating your oven to 300°F (150°C). Rinse the corned beef to remove excess brine—this is optional, but it helps reduce the saltiness. Pat it dry with a paper towel.
In a bowl, mix together the stone-ground mustard, brown sugar, and the spice packet that came with the corned beef.
Spread the mixture over the top of the corned beef with the fat side up, then place it in a Dutch oven or roasting pan. Pour the broth around the corned beef being careful not to wash off the mustard. Cover and put in the oven for about one hour of cooking time per pound. (Please note that cooking times will vary based on your oven as well as the size of the corned beef). *See notes below*
With about an hour and a half left in cooking time, prepare your vegetables. Place the quartered potatoes, chopped carrots, and quartered onion on a baking sheet. Melt the butter and drizzle it over the vegetables. Season them with the steak blend seasoning.
Increase the oven temperature to 350°F (175°C) and place the vegetables in there.
With 45 minutes left, add the quartered cabbage to the pot with the corned beef. Sprinkle with a little more steak seasoning for extra flavor.
Once everything is cooked, toss the vegetables and let the corned beef rest for about 10 minutes. Remember to slice the corned beef against the grain for the best texture.
Transfer to a serving dish and enjoy!