Heat the olive oil or butter in a nonreactive 10- or 9-inch skillet, preferably nonstick, over medium-high heat. Add the onion, bell peppers, and garlic and sauté, tossing with a wooden spoon, until the vegetables are crisp-tender, about 6 minutes. Add the tomatoes, sprinkle with salt, and cook, stirring and tossing, until the liquid evaporates somewhat, 4 or 5 minutes.