Preheat oven to 400 degrees F. Grease a 9x13-inch baking dish; set aside.
Prepare the stuffing according to package directions; set aside.
Cook the sausage over medium heat in a large skillet, breaking it up as it cooks. Add the scallions and cook until the meat is no longer pink. Add the garlic and cook for another minute, or until the garlic is fragrant. Drain any fat from the pan.
Add the prepared stuffing and the spinach to the skillet with the sausage and mix together until thoroughly combined. Transfer to the prepared baking dish and sprinkle with the Monterey Jack cheese.
In a small bowl, whisk together the half-and-half and eggs and pour evenly over sausage mixture. Rap the dish against the counter once or twice to make sure the liquid is evenly distributed.
Bake for 30 minutes, or until a thermometer inserted in the center reads 160 degrees F. Sprinkle with Parmesan cheese and bake for 5 more minutes, or until bubbly. Let sit for 10 minutes before serving.