Place the corned beef with the fat side down into your slow cooker. Add the vegetables and black peppercorns and bay leaves. Fill the pot with enough water to just cover the corned beef. Cover and let slow cook on a low setting for 7 to 8 hours until the corn beef pulls away with a fork.
Carefully remove the meat from the pot, as it will be extremely hot and tender. Set on a cutting board, and allow it to rest until it firms up a bit, about 10 minutes. Slice or shred to put inside your tortilla. Do not discard the cooking liquid, as you are going to want it to cook your cabbage.
In a media sauce pan, strain out the vegetables and pour in the broth from the slow cooker. Add cabbage and let simmer for 10 minutes. Remove cabbage from liquid and drain.
For the cream sauce, mix the media crema with the prepared horse-radish. Mix in your lemon juice to taste as well.
Warm a corn tortilla. Place about 3 ounces of the corned beef in the middle. Top the meat with a spoonful of cabbage. Garnish with onions, cilantro, and avocado. Place a dollop of the horse-radish cream sauce on top along with the slices of jalapeños.