Stir-fry the beaten eggs.
Add the 2 packages of udon noodles. Stir-fry the noodles over medium heat while drizzling in a tiny bit of water and loosening the noodles.
Reduce heat to low mix in the canned tuna.
Add the natto, then 4 tablespoons of mentsuyu. If the natto comes with its own sauce, add that too.
Add the desired amount of mayonnaise and stir.
Serve it on a plate and it is done. Sprinkle on some bonito flakes or shredded nori seaweed and enjoy.