Preheat oven to 400 degrees F and line a baking sheet with aluminum foil.
Cut sweetpotatoes into cubes and transfer to the prepared baking sheet. Drizzle with 2 tablespoons sesame oil and 1½ tablespoons soy sauce. Bake for 18–20 minutes, or until soft when pierced with a fork.
Cut tuna into cubes and place in a medium bowl. Add rice vinegar, remaining 1 tablespoon sesame oil, and 1 tablespoon soy sauce, toss to coat.
To assemble the bowls, evenly divide brown rice, marinated tuna, roasted sweetpotatoes, edamame, carrots, avocados, and green onions among four bowls.
Drizzle with spicy mayo and sesame seeds, if desired.