Combine mayonnaise, sriracha, and 1 teaspoon vinegar in bowl; set aside. In medium bowl, whisk soy sauce, ginger, and remaining 1 tablespoon vinegar together; set aside.

Pulse cauliflower in food processor until chopped into ¼- to ⅛-inch pieces, 6 to 8 pulses, scraping down sides of bowl as needed.

Bring ¼ cup water to simmer in large saucepan over medium-high heat. Stir in cauliflower. Reduce heat to medium-low, cover, and cook, stirring occasionally, until cauliflower is tender, about 12 minutes. Stir in 2 tablespoons oil and half of soy sauce mixture and continue to cook, uncovered, stirring occasionally, until cauliflower rice is almost completely dry, about 3 minutes. Divide cauliflower mixture evenly between individual serving bowls; set aside.

Pat tuna dry with paper towels and sprinkle with salt and pepper. Heat 1 tablespoon oil in 12-inch nonstick skillet over medium-high heat until just smoking. Cook steak until opaque at perimeter, translucent red at center when checked with tip of paring knife, and registers 110 degrees (for rare), or reddish pink at center when checked with tip of paring knife and registers 125 degrees (for medium-rare), 1 to 2 minutes per side. Transfer tuna to cutting board and immediately cut into 1-inch pieces.

Add cucumber and remaining 1 tablespoon oil to bowl with remaining soy sauce mixture and toss to coat. Top prepared serving bowls evenly with tuna, cucumber, and avocado. Drizzle with mayonnaise sauce and sprinkle with scallion; nori, if using; and sesame seeds. Serve.