Remove seeds and ribs from peppers; cut lengthwise into strips and halve crosswise.
Heat oil in a large nonstick skillet over medium heat. Add bell peppers and onion; season with salt and pepper. Cook, stirring occasionally, until peppers are just tender, about 10 minutes.
If you like, you can toss in some fresh chopped herbs, such as basil, dill, or parsley, just before serving.