Place 2 cups of water in the crockpot. Add the beef brisket, fat side up.
Peel and mince the garlic cloves and sprinkle them over the brisket along with the 2 teaspoons of whole black peppercorns.
If your corned beef came with a spice packet, sprinkle it over the brisket. If it did not include a spice packet, or if you would like to add a little more flavor, add 1½ teaspoons of pickling spice. I usually use both.
Cover and cook in the crockpot for 7–9 hours on low or 4–6 hours on high.NOTE: The corned beef can also beplaced in a pan or dutch oven, covered, and braised in a 350℉ oven for 3-4 hours or until fork teder.
Prepare the Vegetables:
About 2 hours before the corned beef is done, wash and cut 1½ pounds of red potatoes into evenly sized pieces and place them on a baking sheet.(I usually cut up 2–2½ pounds and double the butter sauce so we have leftovers.)
Preheat the oven to 350°F.
Clean, core, and cut the cabbage into 5–6 wedges and place them on a second baking sheet.
Slice the green onions.
In a medium glass bowl or 4-cup measuring cup, melt 6 tablespoons of butter. Stir in the green onions, ½ cup prepared horseradish, ½ teaspoon freshly cracked pepper, and ¼ teaspoon salt.
Pour half of the butter mixture over the potatoes and toss until coated. Pour the remaining mixture over the cabbage wedges. Cover both pans loosely with foil.
Place the vegetables in the oven and roast for 55 minutes. Remove the foil and continue roasting 15–20 minutes longer, or until the potatoes are fork-tender and the cabbage is cooked to your preference.
Finish the Corned Beef:
While the vegetables are roasting, mix 2 tablespoons orange marmalade with 2 teaspoons Dijon mustard to make a glaze.
Once the vegetables are fully roasted and the brisket is cooked, cover the vegetables to keep them warm. Set the oven to High Broil.
Place the brisket on a baking sheet and brush the glaze over the top. Place it in the oven 3–4 inches from the heat and broil for 2–3 minutes, or until the glaze becomes bubbly and lightly browned.
Remove from the oven, slice the brisket, and serve with the roasted vegetables and rye bread.