This colorful, healthy yet tasty all-vegetable mixture is a great refrigerator staple for using in your bentos, and is very adaptable. Depending on the flavors you can add later, it can taste Italian, Japanese, Chinese, or whatever suits your needs.
It's a mixture of thinly siiced onions, sweet peppers and a little garlic, sautéed over a fairly low heat until it's quite limp. It's only seasoned with salt, so that it's fairly neutral. You can then turn it more Mediterranean by adding some basil and oregano for example, or Japanese by adding soy sauce, or add some oyster sauce.