Heat the water in a large pot until almost boiling (Note 4), add the salt.
Slowly add the polenta while whisking all the time. Be careful when the polenta starts to bubble; if the bubbles are too big, you might get burned – so turn the heat down (and wear mittens) if that happens.
Cook: Once the polenta is incorporated, continue stirring or whisking shortly to ensure no lumps. Turn the heat down, cover the pot, but leave a crack open. Stir every 2-3 minutes or so with a wooden spoon, but there is no need to stir continuously.
Cooking time: Read the cooking instructions on the polenta packet to know how long the cornmeal needs to be cooked. I cook the polenta made with coarse cornmeal for 40 to 50 minutes and the polenta made with medium cornmeal for about 15 minutes.
While the polenta cooks, make the sauce.
The spinach should be defrosted. Squeeze it well to remove the excess water.
Melt the butter in a medium saucepan.
Sprinkle the flour on top and roast it gently, stirring continuously. It should get only very lightly golden.
Slowly, start adding the milk while whisking continuously to avoid the formation of clumps.
Let bubble gently until the sauce thickens; 1-2 minutes should do it.
Add the squeezed spinach and combine.
Adjust the taste with nutmeg, salt, and pepper.
Assemble polenta al forno
Preheat the oven to 400 degrees Fahrenheit/ 200 degrees Celsius. Butter the baking dish generously.
Grate the cheese. Reserve 2 tablespoons of fontina and about 1/3 of the Parmesan for sprinkling on top.
Transfer ½ of the cooked polenta to the baking dish.
Cover with the spinach sauce.
Sprinkle with the grated cheese, except for the reserved amount.
Cover with the remaining polenta.
Sprinkle with the remaining cheese.
Bake for 25-30 minutes until golden. Serve immediately.
Weigh the spinach before defrosting. Squeeze it well once defrosted.
You can use about 1- 1.3 lb/ 500-600 g. Wash it thoroughly and add it to a large pan with shaking it dry. Let it wilt entirely in the pan, drain it to remove the excess water, and add it to the sauce.
To make the dish vegetarian, use vegetarian hard cheese.
Heat the water in a large pot but don't let it come to a boil (it helps against the formation of lumps).