Equipment: a 9-inch cake pan or spring form pan or a square 8x12” pan
Place ricotta in a cheese cloth or fine mesh strainer and drain overnight or up to 24 hours.
Bring stock to a low boil. Whisk in polenta and let it get very thick and fairly tight, 3-4 minutes. Stir in Parm and season with salt, pepper and nutmeg to taste. Cool polenta slightly to handle.
Brush cake pan or baking dish with olive oil and line pan bottom and sides with polenta, forming a deep-dish crust.
Preheat oven to 425°F.
Heat a skillet with 2 tablespoons olive oil over medium-high heat. Add sausage; brown and crumble, and remove. Add a little more oil and brown mushrooms. Season with salt, pepper and thyme. Add garlic, stir a minute more.
Top polenta pizza crust with the drained ricotta and top with half the sausage and mushrooms. Sprinkle all of the mozzarella over the mushrooms then top with remaining sausage crumbles and mushrooms again. Bake until brown and bubbly, 30 minutes.