Prepare the sushi rice: Rinse rice until water runs clear. Cook with 2 ½ cups water in a rice cooker or stovetop. Mix rice vinegar, sugar, and salt; fold into cooked rice. Cool to room temperature.
Prep fillings: Thinly slice salmon or tuna, avocado, and cucumber. Soften cream cheese if using.
Assemble the cake: Line a 6-8 inch round cake pan with plastic wrap. Spread 1 cup rice evenly, then layer avocado and cucumber. Add cream cheese if desired. Add another rice layer, then salmon or crab. Repeat with rice and fillings.
Chill: Cover and refrigerate for 30 minutes to set.
Unmold and decorate: Remove cake using plastic wrap, transfer to a plate, and top with sesame seeds, tobiko, nori, and microgreens.
Serve: Slice into 10 wedges and serve with soy sauce, wasabi, pickled ginger, and spicy mayo.