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.