In a nonstick skillet over medium heat, toast the panko breadcrumbs. Move them around constantly with a spatula until they turn a golden brown color and become fragrant. Remove from heat and set aside.
Peel the cucumber and avocado. Slice the cucumber, avocado and imitation crab meat into long, thin strips. Set aside.
Make the spicy mayo: combine mayonnaise and sriracha in a small bowl. Set aside.
Lay the bamboo mat on a flat surface. Cover the bamboo mat with a layer of plastic wrap. Press 1 1/3 cups of rice down into a rectangular shape, leaving a perimeter of bamboo mat showing.
Cover the rice with another piece of plastic wrap. Use a rolling pin to smooth and flatten the rice. Remove the plastic wrap and lay a piece of nori on top of the rice (shiny side down).
Spread a thin layer of spicy mayo across the center of the nori sheet. Arrange strips of cucumber, avocado and imitation crab meat over the spicy mayo.
Starting from the bottom, holding on to the bamboo roller, fold the roll up and over the fillings. Press it down firmly. Rotate the roll 90 degrees and repeat the same action, folding the roll up and over the fillings. Press firmly.
Slowly release the bamboo map and unwrap the sushi roll from the plastic wrap.
Spread the toasted panko on a plate. Carefully roll the sushi roll in the bread crumbs. Wet your sharpest knife with water and gently slice the roll into 8 pieces. Drizzle with additional spicy mayo on top of the roll and sprinkle with roasted sesame seeds, if desired.