Cook the sushi rice with the water in a rice cooker. When the rice is nearly finished, stir in the rice vinegar, salt and sugar. Mix well, fluffing the rice with a fork. When the rice cooker turns off, remove the lid to let the rice cool a bit. 

Combine the marinade ingredients in a small bowl. Slice the stems of the mushrooms into ½- to ¾-inch (13- to 19-mm) rounds, like scallops. Roughly chop the caps until they resemble crab meat. Add the mushrooms to the marinade, stir to coat, and set aside for 5 minutes. 

Slice the cucumbers in half, then cut them into ½-inch (13-mm) pieces. Slice the watermelon radish into thin pieces. 

Put a nonstick pan over medium-high heat. Transfer the mushrooms to the pan with a slotted spoon and reserve the marinade. Try to keep the stems on one side of the pan and the tops on the other. Cook the stems to a deep golden brown, about 3 minutes on each side. Cook the tops for 5 to 6 minutes, stirring occasionally. 

Chop the green onions. Slice the avocado. Mix the mayo with the sriracha and sesame oil in a bowl. Add the chopped mushroom tops to the mayo mixture and mix well to make the vegan dynamite. 

Divide the rice among 2 or 3 bowls and top each with a generous scoop of the dynamite mixture. Arrange the scalloped mushrooms, avocado, cucumber slices, green onions and watermelon radish on top. Finish with a drizzle of eel sauce, pickled ginger and black sesame seeds.