"Adapted from Deb at SmittenKitchen, I have made this several times and my husband, though picky about vegetables and vegetarian dishes, always goes back for seconds. It's a great light weekday meal that can be prepared in the morning, and is easy on the wallet. I like it best served atop toasted dense artisan bread with a poached egg, a splash of white wine vinegar, and a few grinds of black pepper. A little freshly grated Parmesan doesn't hurt, either."
The night before cooking, pour beans and vinegar into slow cooker ceramic, adding enough filtered water to sufficiently cover beans that will double in size. Soak overnight. This increases the digestibility and nutrient availability in the beans, and reduces the well-known unpleasant side effect of them.
The morning after, drain and rinse beans, and add back to ceramic. Pour in the stock, tomato puree, and minced garlic. Turn on slow cooker's heat to High.
In a 10" cast iron skillet, melt the butter and saute the shallots, carrots, and celery over medium-high heat until soft and onions are translucent. Add the wine, scrape up any browned bits on the bottom of the skillet, and cook until wine is reduced by about three-fourths. Remove from heat and add it all to beans in slow cooker. Stir everything to combine.
Toss in the thyme sprigs, bay leaf, salt, and several grinds of pepper into the stew; cover, and cook on High 5-6 hours, then Low until you are ready to eat. Ten minutes before serving, turn the slow cooker off. Stir in the chopped spinach; stew is done when it is wilted and you have tasted and adjusted the seasonings to your liking. Serve with toasty bread.