Dice one onion and set aside. Quarter the other onion and place in a four-quart pot. Add water, carrots, celery, sweet potato, and salt and pepper. Bring to a boil.

Reduce heat; cover and simmer for 20 minutes or until carrots are soft. Cool slightly. Puree with an immersion blender or in a food processor.

Meanwhile, heat the olive oil and butter in a large frying pan. Add the reserved onion and the zucchini and sauté for 10–15 minutes. Add to pureed soup. Add the diced tomato. Cover and simmer for 10 minutes.

Add milk and parsley and stir until heated through. Remove from heat. Taste and adjust seasoning, if desired.