Place the rice-cracker mix in a food processor and process into fine crumbs. Place the crumbs in a shallow bowl.
Spread the tuna steaks liberally with the mustard on all sides, using 3 of the 4 tablespoons, then coat the steaks, 1 at a time, in the crumbs.
In a large nonstick skillet, heat 2 tablespoons of the olive oil over medium-high heat until simmering. Sear the steaks until they reach the desired doneness, about 3 minutes per side for medium-rare; set aside.
Peel the apples, then coarsely grate them into a medium bowl. Add the carrots and cabbage. In a small bowl, whisk together the remaining 3 tablespoons of the olive oil and 1 tablespoon of the mustard, plus the vinegar, sugar and salt. Pour over the slaw and toss.
Slice each tuna steak into 1/4-inch slices and serve with the apple slaw.