Begin by selecting high-quality, sashimi-grade tuna steaks from your local market. Ensure the tuna is fresh, suitable for raw consumption. Gently clean the tuna under cold, running water, pat dry with paper towels.

In a mixing bowl, combine mirin, soy sauce, a custom spicy sauce blend (to replicate Wagamama's unique flavor), sesame oil, and a splash of sake. Season with a pinch of salt, chili flakes, and garlic powder according to your taste preferences. Stir well to ensure all ingredients are fully incorporated.

Slice the tuna into bite-sized cubes or thin sashimi slices, depending on your serving preference. Submerge the tuna pieces in the prepared marinade, ensuring each piece is thoroughly coated. Cover the bowl and let it marinate in the refrigerator for about 30 minutes to allow the flavors to meld.

Soak the sushi rice in water for 30 minutes before draining and rinsing it under cold water until the water runs clear. Cook the rice according to the package instructions or your rice cooker's settings until it is sticky and tender. Season the cooked rice with a mixture of rice vinegar, sugar, and salt to taste, then allow it to cool to room temperature.

Slice cucumber, onions, and ginger into fine julienne strips. Consider adding thinly sliced avocado for an extra layer of texture and richness to the dish. Ensure all vegetables are prepped and ready for assembly.

In a separate bowl, gently mix the seasoned sushi rice with the prepared vegetables, carefully folding to combine without crushing the rice grains. Adjust the seasoning with additional soy sauce and teriyaki sauce for added depth of flavor.

If desired, lightly beat an egg and cook it in a hot, non-stick pan to form a soft, slightly runny omelet. This can either be draped over the spicy tuna and rice mixture or served alongside as an accompaniment.

Enhance the presentation with additional garnishes such as slices of avocado, a sprinkle of finely chopped spring onions, sesame seeds, or shredded nori strips. These elements add both visual appeal and a variety of textures to the dish. Serve immediately to enjoy the freshness and vibrant flavors.