Place porcini in a small bowl. Add 1 cup boiling water and let soak at least 20 minutes.
Meanwhile, heat the oil in a heavy Dutch oven or large saucepan over medium heat. Add the onion, bell pepper, rosemary and red pepper flakes. Sprinkle with salt and pepper. Sauté until beginning to brown, stirring frequently, about 8 minutes. Add the sliced mushrooms and sauté until beginning to soften, about 5 minutes.
Add the beans with their liquid and 1 cup broth or water to the pot. Drain the porcini, reserving the soaking liquid. Chop the porcini and add to the pot. Add the soaking liquid. Bring the stew to a boil. Reduce the heat and simmer for about 10 minutes to blend flavors.
Add the spinach to the stew and simmer just until wilted. Season to taste with salt and pepper. Thin to desired consistency with more broth or water. (Can be refrigerated for 3 days. Rewarm before serving, thinning with more broth or water if desired.)
Heat a thin film of oil in a heavy medium skillet. Add the sausage and sauté until nicely browned, about 5 minutes.
Spoon the stew into bowls. Sprinkle the sausage over and serve.