Put the corned beef in a large pot and cover with cold water by 1 inch. Bring to a boil over high heat, then cover, reduce the heat to medium low and simmer until tender, 2 1/2 to 3 hours, adding more water if needed to keep the beef submerged. Remove from the water and let cool 30 minutes, then cut into 1-inch-thick slices.
Meanwhile, make the spice rub: Whisk the chile powder, paprika, oregano, coriander, mustard powder, cumin, chile de arbol and 1 teaspoon each salt and pepper. Set aside.
Make the sauce: Whisk the dijon and whole-grain mustards, the honey and horseradish in a bowl; season with salt and pepper. Let the sauce sit at room temperature at least 30 minutes to allow the flavors to meld.
Spread the spice rub on a plate. Dredge one side of each slice of beef in the rub and tap off the excess. Heat a few tablespoons canola oil in a large nonstick saute pan until it begins to shimmer. A few slices at a time, add the beef, rub-side down, and cook until a golden-brown crust has formed, about 2 minutes. Flip the slices and continue cooking another minute or two, adding more oil as needed. Serve with cabbage and the sauce.
Photograph by Ryan Liebe
Recipe courtesy of Bobby Flay for Food Network Magazine