Sprinkle the corned beef on both sides with the cayenne pepper, seasoned salt, a pinch of Himalayan salt and 1/2 teaspoon black pepper.
Heat a large Dutch oven over high heat and coat with the grapeseed oil. Sear the corned beef until golden brown, 5 minutes per side. Add the garlic, carrots, celery and onions and cook for 5 minutes. Add the beef stock and cover the pot. Turn the heat down very low and let the corned beef cook slowly for 2 1/2 hours.
Remove the corned beef from the pot and let rest for 5 minutes before slicing.
Meanwhile, add 1 cup of the corned beef juice from the pot to a saute pan over low heat. Mix the flour with 1/2 cup hot water and add to the pan. Cook, stirring, until the gravy thickens, a few minutes.
Serve the corned beef and gravy over your favorite rice.