1. Cook the macaroni in a pot of salted boiling water until al dente, about 8 minutes. Drain and place in a large bowl. Add the spinach and toss to combine. Set aside.
2. Heat 1 tablespoon of the oil in a medium-size skillet over medium heat, add the onion, cover, and cook until softened, about 5 minutes. Set aside.
3. Grind the cashews to a powder in a blender or food processor. Add 1 cup of the water and blend until smooth. Add the onion, beans, miso paste, if using, the remaining ¾ cup water, the lemon juice, mustard, cayenne, and nutmeg and season with salt. (The amount of salt you need depends on whether or not you use the miso paste, which tends to be salty.) Blend until smooth, then taste to adjust the seasonings. Pour the sauce over the macaroni and spinach and mix well.
4. Transfer the mixture to a lightly oiled 4-quart slow cooker. Cover and cook on Low for 3 hours.
5. Close to serving time, heat the remaining 1 tablespoon oil in a small skillet over medium heat. Add the bread crumbs, stirring to coat them with the oil. Cook, stirring, until lightly toasted, 3 to 4 minutes. Remove from the heat and set aside. When ready to serve, sprinkle the toasted crumbs on top of the casserole.