Mince the onion. Put it into a heatproof container and microwave for about 90 seconds. It's ready to go if it gets really hot.
Mix together the onion, tuna, mayonnaise, natto (with the sauce), and black pepper.
Spread the oil in a heated frying pan. Crack the eggs into the pan.
Stir the eggs quickly. Place the cheese on top of the soft-set eggs. Then put the mixed ingredients (Step 2) on top and roll the omelet up to finish.
Story Behind this Recipe
This is the omelet version of "Seriously Good!! Aburaage Filled with Natto, Tuna and Mayonnaise" (Recipe ID: 719051)!
The reason why you cook the onions in the microwave to start is because the omelet doesn't cook through. You add the cheese first and separately from the other ingredients so that it will melt easily. I think that adding black pepper makes the omelet slightly Western-styled.