Preheat the oven to 350°F.
Lightly grease the casserole dish with olive oil.
Prepare pasta according to package directions. Drain. Set aside.
Place a heavy skillet over medium heat.
Add butter and allow to melt, but not brown.
Reduce heat to medium-low.
Add onions. Cook slowly until they are golden brown, about 15 - 20 minutes.
Place prepared pasta, caramelized onions, sausage rounds, spinach, sage, minced garlic and mozzarella cheese into a large mixing bowl. Add salt and pepper. Toss to combine. Taste. Add more salt and pepper, to taste, if needed.
Pour mixture into prepared casserole dish. Spread to fill dish evenly.
Sprinkle Parmesan cheese on top. (At this point the casserole can be stored, tightly covered, in the refrigerator, until ready to cook, up to 24 hours. Be sure to allow the casserole to come up to room temperature before placing in the oven for proper baking.)
Place in preheated oven and bake for 30 minutes, or until top is lightly browned and mozz cheese is bubbling.
Remove from oven and let cool for about 5 minutes before serving.