Preheat a large skillet over med-high heat with 2 tablespoons olive oil and butter.
When the butter has melted, add the sausage, breaking it up with a wooden spoon.
While the sausage is starting to brown, cut the portobello mushroom in half and then thinly slice and add to the sausage.
Chop and add to the pan as you go—the onion, garlic, and rosemary.
Season with red pepper flakes, salt, and pepper.
Once everything is in the pan, the sausage is browned, and the mushrooms and onions have wilted, add in the chicken stock and lemon juice.
Continue to cook over high heat until the liquids have evaporated—2 to 3 minutes.
While the sausage and mushrooms are cooking down in the liquid—combine the ricotta cheese, egg, lemon zest, Parmigiano, and bread crumbs in a bowl.
Brush an 8-inch springform pan or an 8x8 inch baking dish with a little olive oil; place 7 disks of the polenta on the bottom of the dish.
If they overlap a little, that is fine (it will vary depending on what kind of dish you are using).
Divide the sausage and mushroom mixture into 3 even portions; do the same with the ricotta mixture.
Using the back of a spoon, spread 1/3 of the ricotta mixture on top of the polenta (don’t make yourself crazy doing this; just mush it out over the polenta).
Top the ricotta with 1/3 of the sausage mixture.
Repeat layers 2 more times, starting with polenta and finishing with sausage mixture.
Sprinkle the top with mozzarella.
Cover the baking dish with foil; place in oven; bake for 15-18 minutes.
To make sure it is cooked through, insert a knife into the center of the dish, wait a few seconds, then remove it.
Touch the knife to your fingertips and if it is hot, the lasagna is done.
Remove from the springform and serve.
*Sweet and Savory Polenta “Lasagna”—add ½ cup golden raisins and ½ cup pine nuts at the same time you add the chicken broth and lemon juice to the sausage.
*Sausage, Mushroom, and Olive Polenta—add 4 rounded tablespoons black olive or green olive tapenade to the ricotta cheese along with the egg, lemon zest, parmigiano, and bread crumbs.
*Sausage, Mushroom, and Pesto Polenta—add 4 tablespoons quality pesto to the ricotta along with the egg, lemon zest, parmigiano, and bread crumbs.
*Super Mushroom Polenta—omit sweet Italian sausage, swap dry white wine for the chicken broth, add 2 additional portobello mushrooms and 1 jarred roasted red pepper, drained and chopped; add the 2 extra mushrooms along with the original mushroom quantity; add the roasted red pepper to the ricotta mixture.