Add a generous amount of oil and/or butter in the bottom of a pot big enough to hold all your ingredients with room to spare. Add onions first, allow them to sauté without browning while you chop the carrots, then add the chopped carrots while you chop the celery, then add the celery. Add a healthy pinch of salt as you sauté.If you like, you can also add some chopped pancetta to this base. If so, add it first. Add non leafy vegetables in season, one by one, to the pot, allowing each ingredient to be well coated with the oil and absorb the flavor of all the vegetables that have preceded it.
Add leafy vegetables that you want to, cut up into strips, proceeding as for Step 2 above.
Add water (or broth) to cover, bring to a gentle simmer and allow to continue simmering for 45-60 minutes, or until all the vegetables are almost meltingly soft and the soup is getting quite thick.
Add one or more legumes, either pre-cooked or canned. Canned legumes should be drained and rinsed beforehand. Cook for another 20-30 minutes, adding more water if the soup is getting too thick. As they simmer, crush some of the vegetables against the side of the pot.
You can serve the soup immediately, but it is much better made ahead and served the next day, or at least after a few hours’ rest.