Rinse sushi rice under cold water until water runs clear.
Combine rice and water in a saucepan, bring to a boil, then reduce heat to low, cover, and simmer for 15 minutes.
Remove from heat and let sit covered for 10 minutes, then fluff gently with a fork and let cool slightly.
Lightly grease a muffin tin and press about 2 tablespoons of warm rice into each cup, forming a compact base with raised edges.
Allow rice cups to cool completely to hold their shape.
Toss chopped veggies and mukimame with soy sauce or chosen sauce.
Gently fold diced avocado into the veggie mixture just before assembling.
Whisk together mayonnaise, Sriracha, coconut aminos or soy sauce, honey, and sesame oil until smooth to make spicy mayo.
Spoon vegetable mixture into each rice cup, drizzle or pipe spicy mayo on top, then sprinkle with black sesame seeds.