Heat skillet over medium-high heat. Add oil. Add onions and let saute for 3-4 minutes. Add chicken breast, salt, and pepper and saute for 8 minutes. Add carrots and let saute for 2 more minutes. If using red pepper slices, add them to the skillet.
Add potatoes and water and cover. Let simmer, covered, for 6 minutes. If using zucchini instead of potatoes, don't add the water. Add the zucchini and saute for only 2 minutes.