Make the dipping sauce: Combine mirin, miso, ginger, carrot juice, orange juice, soy sauce, vinegar, red-pepper flakes, and sesame oil in a small bowl, whisking until miso dissolves. Sprinkle with sliced scallion.
Make the tuna: Combine sesame seeds in a shallow dish. Press tuna pieces into seeds, turning to coat each side. Heat oil in a medium saute pan over medium-high heat. Add tuna, and cook each side until white seeds are golden, about 20 seconds per side. Transfer tuna to a paper-towel-lined plate for 1 minute, then cut each piece into 1/4-inch-thick slices. Serve with dipping sauce.