Combine the 2 tbsp of oil, vinegar, ½ tsp salt, mustard, and honey in a bowl. Add in the fennel, beets, turnips, and the apple. Toss until coated. Sprinkle the salad with sesame seeds. Heat the remaining oil in a cast iron skillet on high. Sprinkle the pepper and remaining salt on the tuna. Cook for 90 seconds on each side for rare. Remove the tuna from the pan, thinly slice and place the tuna on top of the salad. Place the fennel fronds on top of the salad.