Clean and cut the onion, carrot, green onion, celery, and tomato into bite-sized pieces.
Drizzle olive oil in a pot and sauté the onion, green onion, and celery over low heat.
Add the carrot and tomato, and sauté together.
Add water and bring to a boil over high heat.
Once it starts boiling, reduce to low heat and simmer until the vegetables become tender.
Season with salt and pepper.