In a large skillet, heat the oil over medium heat.
Add the onion and garlic; cook, stirring often, until the onion is softened, about 3 minutes.
Add the sausage and Italian seasoning; increase the heat to med-high; cook, stirring often and breaking up the meat with a spoon, until the sausage is cooked through, about 10 minutes.
Scrape the sausage mixture into a large bowl.
Mix in the breadcrumbs, ricotta cheese, spinach, Parmesan cheese, eggs, nutmeg, salt, and pepper.
Transfer mixture to a lightly buttered shallow baking dish; bake, uncovered, in a preheated 350° oven for 30 minutes.