• 1 c. mozzarella cheese, shredded (about 4 oz.)
Heat water and milk to a boil in a large saucepan over medium-high heat; stir in 1 teaspoon salt. Very slowly pour polenta into boiling liquid while stirring constantly in a circular motion with a wooden spoon. Reduce heat to a gentle simmer; partially cover and cook, stirring often and making sure to scrape bottom and sides of pot clean. Cook until polenta no longer has a raw cornmeal taste, all liquid has been absorbed and mixture has a smooth, uniform consistency but is very loose, about 15 minutes.
Remove polenta from heat. Stir in Parmesan cheese and butter; season with salt and pepper to taste. Pour polenta into a 9- by 13-inch baking dish; cool to room temperature, about 30 minutes.
While polenta cools, heat oil in a 12-inch skillet over medium-high heat until shimmering. Add onion and remaining 1/2 teaspoon salt; cook until onion is softened, about 5 minutes. Add sausage. Cook, breaking up with a wooden spoon into large chunks, until meat is lightly browned, about 10 minutes. Stir in garlic, pepper flakes and tomatoes; cook until fragrant, about 10 minutes. Remove from heat and stir in spinach, 1 handful at a time, until wilted. Season with salt and pepper. Cool to room temperature. Spread cooled sausage mixture over cooled polenta. If not baking right away, wrap dish tightly with plastic wrap; refrigerate up to 24 hours.
To finish, unwrap dish and cover tightly with greased aluminum foil. Bake in a 400-degree oven until hot throughout and bubbling at edges, about 30 minutes. Remove foil; sprinkle casserole with mozzarella. Bake uncovered until cheese melts, 10 to 15 minutes. Let cool 10 minutes before serving.