Pat the tuna steaks dry with a paper towel; set aside on a plate. Spread the sesame seeds on another plate in a thin layer.
In a small bowl, whisk together one-third of the tamari, one-third of the sesame oil, and all of the pepper. Drizzle the mixture over the tuna steaks, gently turning them around to coat. Gently dip the steaks, including the sides, in the sesame seeds, pressing gently to ensure they stick and completely cover.
In a small bowl, combine the remaining tamari and sesame oil with the rice vinegar, peanut butter, garlic, ginger and Sriracha. Whisk until smooth.
In a large nonstick or cast-iron skillet, heat the avocado oil over medium-high heat until it shimmers. Add the tuna steaks, and sear each side until the white seeds turn golden, 1 to 1 1/2 minutes per side. Using tongs, gently hold the steaks upright and sear the short sides, about 20 seconds per side. Transfer to a cutting board; let rest.
Using the same skillet over medium-low heat, add the drained noodles and sauce. Toss to combine.
Divide the noodles between dinner plates. Sprinkle with the green onion. Using a sharp chef’s knife, cut the tuna steaks into ½-inch thick slices . Arrange the slices over the noodles and serve.