Heat the remaining 4 tablespoons sesame oil until smoking in a separate saute pan. Remove tuna fillets from the marinade and generously season all sides with salt and pepper. Carefully place the fillets into the hot pan. Sear all sides of the fillets, 2 minutes for rare. Remove from the pan.
To serve, in the center of each plate place some udon noodles. Slice tuna fillets in half, on the bias, and place on udon noodles. Arrange Green Papaya Salad over top of tuna and serve.
Combine soy sauce, ginger, vinegar, and chili sauce in a blender. Puree until smooth. Slowly add in the sesame oil until sauce is emulsified. Season, to taste, with salt and pepper. Store refrigerated for up to 1 week
Combine yuzu juice and brown sugar in a non-reactive saucepan; place over high heat and bring to a boil. Reduce to a syrup consistency. Whisk in yuzu chili paste and togerashi. Store refrigerated for up to 1 month.
Combine ginger ale, water, 1/2 cup lime juice, 2 teaspoons salt in a saucepan. Place over high heat, and bring liquid to a boil. Add green papaya to boiling liquid, and poach for about 8 minutes. Remove papaya from poaching liquid, and set in cooler to chill.
Combine remaining ingredients in a mixing bowl. Add chilled, poached papaya to ingredients, and mix thoroughly. Season, to taste, with salt and pepper.
*Can be found in Asian or specialty food markets.