Place corned beef in a large, heavy pot; cover with water.
Stud peeled onion with whole cloves. Add to the pot with peppercorns, garlic cloves, and chopped jalapeños. Bring to a boil over medium-high heat, reduce to low, cover, and simmer for 2 1/2 hours, or until the meat is tender.
Preheat the oven to 350° F.
Remove the corned beef to a shallow baking dish; reserving the cooking liquid. Sprinkle with salt and pepper then spread orange marmalade on top. Pour the cooking liquid on top and bake for 25 minutes, basting occasionally.
Servings: 8