Cut chicken thighs into 1/2-inch strips. In a bowl combine 2 tbs soy sauce, 1 tbs mirin, 1/2 tsp minced garlic and 1/4 tsp grated ginger. Add chicken, toss to coat, and marinate 15–20 minutes in the fridge.

Prepare batter

In a bowl whisk together 1/2 cup all-purpose flour, 1/4 cup cornstarch, 1 large egg, 1/4 tsp salt and 1/8 tsp black pepper. Gradually add 1/2 cup cold sparkling water while gently mixing just until combined; the batter should be lumpy and cold (do not overmix).

Fry chicken

Heat 2 cups vegetable oil in a heavy skillet or pot to 350°F (175°C). Coat marinated chicken pieces in a light dusting of flour (to help batter adhere), dip into the cold batter, then carefully fry in batches 2–3 minutes per side until golden and cooked through (internal temp 165°F / 74°C). Drain on a wire rack over paper towels. Keep warm.

Sushi Rice & Assembly

Cook sushi rice

Rinse 2 cups sushi rice under cold water until the rinse runs clear. Combine rice and 2.2 cups water in a rice cooker or pot; cook according to rice cooker instructions or bring to a boil, cover, reduce to low and simmer 18 minutes. Let rest covered 10 minutes off heat.

Season rice

While rice cooks, gently warm 3 tbs rice vinegar, 1.5 tbs sugar and 1 tsp salt in a small saucepan just until sugar dissolves. Transfer rice to a large bowl or hangiri; fold in the vinegar mixture with a rice paddle, fanning to cool until slightly sticky and shiny. Keep rice covered with a damp towel until assembly.

Prepare spicy mayo

Mix 1/2 cup Japanese mayonnaise with 2 tsp sriracha. Adjust heat to taste. Reserve 3 tbs for drizzle and keep remainder for filling.

Lay out nori

Place a bamboo sushi mat topped with plastic wrap. Put one nori sheet shiny-side down. With wet hands, spread ~3/4 cup seasoned rice evenly over the nori leaving a 1-inch strip at the far edge.

Assemble roll

Brush a thin line of spicy mayo across the near third of the rice. Place a few strips of fried tempura chicken lengthwise about 1/2 inch from the near edge, add a few cucumber julienne and 2–3 slices of avocado. Use the mat to roll the sushi tightly away from you, applying gentle pressure to form a cylinder. Seal the edge with a little water.

Repeat

Repeat for remaining nori, rice and fillings to make 3–4 rolls depending on thickness desired.

Crunch Topping & Finishing

Toast panko

In a small skillet, heat 1 tbs vegetable oil over medium heat. Add 3/4 cup panko breadcrumbs and toast, stirring frequently, until golden and crunchy, 3–5 minutes. Remove from heat and let cool.

Top and sauce rolls

Using a sharp, wet knife, slice each roll into 8 pieces. Arrange pieces on a platter. Drizzle eel sauce (4 tbs) and reserved spicy mayo over the tops. Sprinkle toasted panko evenly over each piece for crunch and finish with a pinch of toasted sesame seeds.

Serve

Serve immediately with soy sauce, wasabi and pickled ginger on the side. Leftover fried chicken can be kept warm and crisped in a 350°F oven for a few minutes before assembling additional rolls.