Line a 3 1/2- or 4-quart slow cooker with a disposable cooker liner. Combine tomatoes, the boiling water, cornmeal, the 1/2 cup Parmesan, the salt, basil, and crushed red pepper; stir well to combine. Cover and cook on HIGH for 1 1/2 hours or on LOW for 3 hours, until cornmeal is tender.
About 15 minutes before serving, in a large skillet heat reserved oil from tomatoes. Cook and stir mushrooms in hot oil for 5 minutes until tender. Add spinach; cook and stir just until wilted. Season to taste with salt.
Stir polenta, then spoon into bowls;. Top with spinach and mushroom mixture and crumbled Parmesan.