3 tablespoons vegetable oil
2 large red onions, peeled, halved and then into 1/4-inch strips
2 large green bell peppers, cored, seeded and cut into 1/4-inch strips
2 large red bell peppers, cored, seeded, and cut into 1/4-inch strips
Heat the vegetable oil in a large, deep, skillet over medium heat. Stir in the red onions and cook, stirring often, until they are wilted, about 5 minutes.
Add the green peppers and cook until they are wilted enough to make room for the red peppers.
Add the red peppers, season the vegetables lightly with salt and adjust the heat to low.
Cook, stirring often, until tender, about 20 minutes.
The vegetables can be prepared several hours in advance. Reheat them over low heat just before serving.