Preheat your oven to 350.
Heat a pan over medium high heat. Add the sausage and cook until cooked through and brown, breaking up with a wooden spoon. Remove the browned sausage and set aside.
To the pan, add a tablespoon of olive oil and the polenta rounds. You'll need to do this in batches. Fry on one side for 2 minutes, flip and fry another 2 minutes.
Pour a little of the marinara in the bottom of an 8×10 baking dish. 9 x 13 works, too, you'll just a little more space around the polenta rounds.
Place 8 rounds in the bottom of the dish, two at a time. Follow with half of the browned sausage and chopped thawed spinach. Layer the remaining 8 polenta slices, and the rest of the spinach and sausage. Pour the rest of the sauce over the dish.
Sprinkle with the grated cheese, bake uncovered for 30-40 minutes, or until the cheese is browned and bubbly.