Preheat the oven to 375F.

To prepare the casserole, heat the butter and olive oil in a large skillet and sauté the sliced mushrooms until soft. Add in the baby spinach and let it cook until wilted. Transfer the veggies to a baking dish.

Thinly slice the Polish Sausage and add them to the same skillet. Lightly cook the sausage for around five minutes, then add it to the baking dish.

Whisk together the eggs, heavy cream, shredded mozzarella cheese, salt and pepper, then pour it over the sausages.

Cover and bake in the oven for 15 minutes, then sprinkle over more cheese, and bake for another 15 minutes uncovered.