Whole-wheat somen noodles are thin, light, delicate, and soft. Similar to vermicelli, these Japanese noodles are often served cold in summer months. They add substance to this crunchy, colorful slaw.
Cook somen noodles in large pot of boiling water until done, 2 to 3 minutes. Meanwhile, in a small bowl whisk together ginger, garlic, red-pepper flakes, sugar, soy sauce, rice vinegar, and oils; set aside. Drain noodles in a colander, rinse with cold water, and shake to drain noodles well. Place noodles in large bowl, toss with scallions and half of dressing, about 1/3 cup. Place on large round serving platter on one-third of the plate and set aside.
Place cabbage, carrots, cucumber, radishes, cilantro leaves, and sesame seeds in the empty bowl and toss with remaining dressing. Place on serving platter adjacent to the noodles, covering another third of the platter; set aside.
Dry tuna with paper towel, rub with oil, season with salt and five-spice powder, if using. Grill or pan-sear tuna over medium-high heat, about 1 1/2 to 3 minutes per side for rare to medium-rare steaks. Cut into 1/4-inch slices and place in a number of rows in fan pattern on remaining third section of serving platter. Place cilantro sprigs in center of platter and serve immediately.
You can eliminate the somen noodles to make a low-carbohydrate version of this recipe (simply add 3 additional cups of cabbage and 1 1/2 extra tablespoons of dressing to the slaw).