Combine shichimi togarashi spices, 2 tsp salt, sesame oil, garlic, soy sauce and mirin in a bowl. Pour 3/4 of the marinade in bowl with pork tenderloin and marinate for 6-12 hours, turning halfway through.
Place remaining marinade in a covered container and refrigerate until ready to use.
Remove tenderloin from bag, let sit at room temperature. Turn grill over medium-high heat, brush with vegetable oil.
Add tenderloin to grill and place in center of grate. Discard bag with marinade. Cover and cook 12-15 minutes, turning every 1-2 minutes until tenderloin reaches an internal temperature of 140 F degrees.
Remove tenderloin from grill and place on a large piece of heavy aluminum foil, folded at the edges. Pour on reserved marinade. Wrap tightly and let rest for 10 minutes. Move to a cutting board and slice.
Serve alongside spicy gomasio soba noodles and enjoy!