Shaving raw root veggies into a side-dish salad is a fantastic approach. They're ready in just a few minutes, their earthy flavors stay vibrant, and a simple vinaigrette tenderizes them while retaining some crunch.
Combine 2 tablespoons oil, vinegar, 1/2 teaspoon salt, mustard, and honey in a large bowl. Add beets, fennel, turnips, and apple; toss to coat. Sprinkle salad with sesame seeds.
Heat remaining 2 tablespoons oil in a cast-iron skillet over high. Sprinkle tuna with pepper and remaining 1/2 teaspoon salt; place in hot pan. Cook 90 seconds on both sides (for rare) or until desired degree of doneness. Remove tuna from pan. Slice thinly, and serve with salad; top with fennel fronds.