Preheat oven to 375 degrees F (190 degrees C). Grease a 9x13-inch baking dish.
Brown turkey sausage and fennel seeds in a large skillet over medium heat, about 10 minutes; drain off excess grease, then add mushrooms and spinach. Cook until the mixture is hot and the juice from the spinach and mushrooms has evaporated,10 to 15 more minutes. Spread the turkey mixture into the prepared baking dish.
In a bowl, mix the butter, Parmesan cheese, half and half cream, Monterey Jack cheese, mustard powder, and vermouth until well combined; pour over the sausage mixture in the baking dish.
Bake in the preheated oven until bubbling and the topping has browned, 20 to 30 minutes. Check after 20 minutes.
