Clean and chop the carrots, celery, onion, and potatoes. Thaw the spinach. Remove the thick central stem from the cabbage and kale and chop the leaves roughly.
Set aside ½ cup of beans and blend the rest together with their water until you get a smooth cream.
In a large pot, drizzle some olive oil and sauté the onion for a couple of minutes, then add the tomato paste, cook for another minute, then add the beans cream, and the whole beans.
Mix with a spoon then add all the other chopped vegetables. Season with salt, black pepper, and thyme.
Add enough water to barely cover all the vegetables and simmer at low heat for about 2 hours. Check it occasionally and add a bit of water if it's too dry.
Halfway through the cooking, rub a garlic clove on the slices of bread. Chop the bread slices in large chunks and add them to the stew. Mix well so they get covered.
Serve the Ribollita in bowl adding a drizzle of olive oil on top.
Notes
Tip: If you have it available, you can also add a Parmesan rind to the stew to give it more depth of flavor.Notes about Bread: The traditional recipe uses Tuscan bread, which is crusty sourdough bread and it's unsalted. But any type of good-quality bread will do, I suggest using crusty sourdough bread. The bread should also be stale, so it will soften in the stew without falling apart. Alternatively, you can toast the bread slices.Notes about Storage: You can keep leftovers in an airtight container in a fridge for up to 1 week or in the freezer for up to 6 months.