Place rice in a fine mesh strainer, then run under cool water to thoroughly rinse. At first, the water will look cloudy or murky, the water should run clear. Cook the rice. Pour sushi vinegar on top of rice, directly in the pot it was cooked in.
Pat the tuna dry on both sides with paper towels, then brush each filet lightly with sesame oil. Season each tuna steak with Kosher salt. Mix black sesame seeds and white white sesame seeds together on a plate or sheet tray. Firmly press the tuna filets into the sesame seeds.
Bring a pan to medium-high heat, then add remaining neutral oil. When just barely smoking, carefully add the tuna fillets, or 1-2 minutes per side for medium. Transfer the seared tuna to a plate. Allow to cool slightly.
Thinly cut tuna across the grain into slices about ¼ -inch thick, then divide between bowls. Drizzle with spicy mayo, then garnish with your favorite toppings. I like to add watercress, scallions, thinly sliced Thai red chiles, cilantro, sesame seeds, and lime wedges. Pickled ginger can be added to the bowls or served on the side. Enjoy hot, room temperature, or cold.