Set the oven to 375°F and microwave the frozen cauliflower according to its package directions, about 3 to 5 minutes. Leave it to cool until you can handle it comfortably.
Use the cooling time to chop the imitation crab by hand or pulse it in a food processor. A finely chopped texture will blend evenly into the creamy filling.
Beat the egg, reserve a portion for the cauliflower, and mix the remainder with the crab, filling portion of Kewpie mayo, softened cream cheese, black sesame seeds, and sriracha in a large bowl. The mixture should look cohesive, without firm pockets of cream cheese.
Cover the bottom of a 9x9-inch baking dish with the seafood mixture. Level it into an even layer so the cauliflower can cover it uniformly.
Gather the cooled cauliflower in a clean kitchen towel and twist firmly to extract its liquid. Keep squeezing until the bundle feels compact and liquid stops dripping.
Warm the sesame oil in a large skillet over medium heat, then add the squeezed cauliflower and optional dashi miso paste. Stir continuously for about 5 minutes to release lingering moisture while keeping the cauliflower pale rather than browned.
Distribute the mayo, furikake, reserved beaten egg, rice vinegar, and sweetener throughout the cauliflower. The seasonings should coat the rice evenly, with no visible streaks of egg.
Cover the seafood with the seasoned cauliflower, then finish with the reserved Kewpie mayo, sriracha, and optional pineapple. Spread the rice to the edges without disturbing the filling underneath.
Place the uncovered dish in the oven for 25 minutes and check that the center is heated through. The creamy topping may bubble around the edges, but edge bubbling alone does not confirm the center is ready.
Scatter the diced cucumber, green onion, and avocado over the finished bake. The fresh toppings should stay cool and distinct against the warm casserole.