TO PREP

1. Bring water and milk to boil in large saucepan over medium-high heat. Stir in 1 teaspoon salt, then very slowly pour polenta into boiling liquid while stirring constantly in circular motion with wooden spoon. Reduce to gentle simmer, cover partially, arud cook, stirring often and making sure to scrape bottom and sides of pot clean, until polenta no longer has raw cornmeal taste, all liquid has been absorbed, and mixture has uniformly smooth but very loose consistency, 15 to 20 minutes.

2. Offheat, stir in Parmesan and butter and season with salt and pepper to taste. Pour polenta into 13 by 9-inch baking dish and let cool to room temperature, about 30 minutes.

3. Meanwhile, heat oil in 12-inch skillet over medium-high heat until shimmering. Add onion and ½ teaspoon salt and cook until softened, about 5 minutes. Add sausage and cook, breaking meat into large chunks with wooden spoon, until lightly browned, about 10 minutes. Stir in garlic and pepper flakes and cook until fragrant, about 30 seconds.

4. Stir in tomatoes and their juice, bring to simmer, and cook, stirring occasionally, until sauce has thickened, about 10 minutes. Off heat, stir in spinach, 1 handful at a time, until wilted. Season with salt and pepper to taste. Let cool to room temperature, about 30 minutes. Spread cooled sausage mixture evenly over cooled polenta.

TO STORE

 5. Wrap dish tightly with plastic wrap and refrigerate for up 24 hours.

 TO FINISH AND SERVE

6. Adjust oven rack to middle position and heat oven to 400 degrees. Unwrap dish and cover tightly with greased aluminum foil. Bake casserole until hot throughout and bubbling around edges, about 30 minutes. Remove foil, sprinkle with mozzarella, and bake, uncovered, until cheese is melted, 10 to 15 minutes.

Let cool for 10 minutes. Serve.