In a small saucepan, combine the soy sauce, mirin, sake, rice vinegar, sugar, grated ginger, and grated garlic. Heat over low heat and stir until the sugar is dissolved. Remove from heat and let the sauce thicken and cool to room temperature
Season tuna with salt and pepper. Heat oil in a nonstick skillet over high heat. Sear tuna 30–60 sec per side, including edges, until browned outside and rare to medium-rare inside. For grill marks, rotate as it sears. Let rest a few minutes.
Using a very sharp knife, slice the seared tuna into thin, even slices. The slices should be about 1/4 inch thick
Slice tuna and arrange on microgreens or arugula. Drizzle with tataki sauce, then garnish with green onions, sesame seeds, and red pepper flakes if desired. Serve immediately.
