The night before, put the dried beans in a bowl and cover them with a couple of inches of cold water. Let soak overnight (at least 8 hours). Using canned beans? Skip this step and the next one (see notes).

The next day, drain and rinse the beans. Put them in a saucepan, cover with fresh water by about 2 inches, and bring to a boil. Lower the heat and simmer, partly covered, for 45 to 60 minutes, until tender. Drain and set aside. (You can do this while the soup simmers.)

Heat the olive oil in a large soup pot or Dutch oven over medium heat. Add the onion and cook, stirring often, for 5 to 6 minutes, until softened and golden.

Add the carrots and celery, season with a pinch of salt and pepper, and cook for 2 minutes, stirring.

Add the potatoes, stir to coat them in the oil, and cook for 2 minutes more.

Stir in the spinach and cook for about 2 minutes, until wilted.

Add the vegetable stock, crushed tomatoes, and thyme sprigs, and season lightly with salt and pepper. Bring to a simmer, then cover, turn the heat to low, and let the soup simmer very gently for about 2 hours, stirring occasionally. Keep it at a bare simmer, not a boil.

Stir in the cooked beans and the broken fettuccine. Simmer, uncovered, for about 20 minutes, stirring now and then so the pasta doesn't stick, until the pasta is tender. Remove the bare thyme stems, then taste and adjust the salt and pepper.

Ladle into bowls and serve hot, with crusty bread on the side.