Add dried chickpeas to a large bowl then add water, covering by 2 inches (they expand a good amount). Let soak overnight then drain well.

Warm olive oil in a large, heavy bottomed pot over medium heat. Add the onion, carrot, celery, and bay, stirring to coat in the oil. Cook until the vegetables are soft, but not brown (6 - 7 minutes). Stir in vegetable stock and scrape the bottom of the pan to lift any brown bits. Transfer mixture to a slow cooker. Add chickpeas, stir, cover and cook on low for 6 hours.

Stir in 1 teaspoon of kosher salt, several turns of black pepper, and the tomatoes, cover and cook for an additional 2 hours. The chickpeas should be tender, but not mushy. Season to taste with additional kosher salt and black pepper.

Remove and discard bay leaves. Scoop chickpeas from the slow cooker using a slotted spoon and place in a serving bowl. Reserve the braising liquid.

While the chickpeas are cooking, prepare the vinaigrette. Add red onion, garlic, red wine vinegar, kosher salt and a few turns of black pepper to a mason jar, shaking to combine. Add extra-virgin olive oil and shake again until the vinaigrette emulsifies (oil and vinegar mix together). Just before you are about to serve, toss baby spinach with vinaigrette.

To serve, spoon chickpeas on to a large, dinner plate. Top with a little braising liquid to moisten. Place spinach salad on the side.