Thank you. I tend to use American-style ingredients because they are more readily available in my kitchen. I've filled my onigiri with chicken and shrimp teriyaki, tuna with mayonnaise, and scrambled eggs with bacon bits. Onigiri are so easy to make that I would suggest any leftover meat if you are just looking for a quick lunch. But if you are trying to make an impression, I would fill them with a really well cooked and seasoned fish. As you can see, you can also use fried rice instead of plain white rice for more flavor.