Heat a large pot over medium heat. Once hot, add olive oil, diced onion, carrots, and chopped fennel or celery. Cook, stirring occasionally, until the vegetables are soft and translucent, about 5 minutes. Add garlic and cook for about 1 more minute until fragrant.
Add cubed potatoes, zucchini, green beans, red pepper flakes, dried basil (optional), and the lesser amount of sea salt. Cook for 5-10 minutes, stirring occasionally, until the vegetables are slightly softened.
Add the crushed tomatoes and cook for another 5 minutes, stirring occasionally. Add the water and bring to a boil. Lower the heat to a simmer and continue cooking for 10 minutes. Add the white beans and cook for another 5-10 minutes, until the potatoes are tender. Taste and adjust as needed, adding more salt for overall flavor or red pepper flakes for heat.
Store leftover soup in an airtight container in the refrigerator for up to 3-4 days or in the freezer for 1 month or longer. Reheat in the microwave or on the stovetop, adding a little water if needed.
Notes
*Nutrition information is a rough estimate calculated with fennel, the lesser amounts where ranges are provided, and without optional ingredients.