Season the cod with salt and pepper.

Julienne the carrot, roughly chop the napa cabbage, and separate the shimeji mushrooms.

Spread 2/3 of the cut vegetables on aluminum foil, place the cod on top, then add the remaining vegetables and drizzle with mayonnaise.

Wrap everything in the aluminum foil, place in a skillet over medium heat, and cover with a lid. Once you hear it simmering, reduce to low heat and cook for 10 minutes.

Drizzle with ponzu sauce just before serving.