Add water, chicken broth, and salt to a large saucepan and bring to a boil. Gradually whisk in polenta until no lumps remain.
Reduce heat to a simmer; whisking often, and cook until polenta starts to thicken, 3 to 4 minutes. Cover the pot and cook for 20 to 30 minutes, or follow package directions, Stir frequently and scrape the bottom to avoid burning, until consistency is creamy and tender. Your timing will depend on the polenta grind, so just keep an eye on it.
Meanwhile, heat olive oil in a skillet over medium heat. Add mushrooms, and sauté for 4 to 5 minutes, stirring occasionally. Add garlic and cook until fragrant; stir in spinach, and cook until wilted, about 3 to 4 minutes; set aside.
Stir butter and Parmesan into cooked polenta until melted. If polenta is a little thick, stir in more water or chicken broth. Stir in spinach-mushroom mixture until everything is well combined. Season with salt and pepper, garnish with grated Parmesan if desired, and serve hot.
