Add zucchini, mushroom and red bell pepper. Cover with lid and set temperature to HIGH (430°F/220°C). When the soup reaches a boil, skim the surface if needed, reduce heat to SIMMER (280°F/140°C) and cook for an additional 6 minutes. Taste soup and adjust with salt if necessary.
Set skillet to KEEP WARM, take cooked ingredients from the skillet and serve.