Prepare the base by cooking the white rice according to package instructions, then steam the cauliflower rice and fluff them together in a bowl.
Using a very sharp knife, cut the sushi-grade ahi tuna into bite-sized 1/2-inch cubes and place them in a medium glass mixing bowl.
In a small ramekin, whisk together the tamari, sesame oil, rice vinegar, and freshly grated ginger until emulsified.
Pour the marinade over the tuna cubes and gently toss to coat; let it sit for 5 minutes to allow the flavors to penetrate.
Slice the cucumber into thin rounds and dice the avocado into small chunks.
Assemble the bowl by placing the rice mixture at the bottom, then topping with the marinated tuna, edamame, cucumber, and avocado.
Garnish with sliced green onions, sesame seeds, and a pinch of sea salt before serving immediately.