Preheat oven to 350°F (175°C) and lightly grease a 9x13-inch baking dish

Cook manicotti in salted boiling water until slightly underdone, then drain and cool

Heat olive oil in a skillet over medium heat and cook sausage until browned and crumbled

Add diced onion and cook until softened, then stir in garlic and cook briefly

Remove from heat and allow mixture to cool slightly

In a large bowl, combine ricotta, egg, mozzarella, Parmesan, parsley, and seasonings

Mix in the cooked sausage mixture until well combined

Transfer filling into a piping bag or zip-top bag with the corner cut

Spread 1 cup marinara sauce evenly on the bottom of the baking dish

Fill each manicotti tube with the prepared mixture and arrange in the dish

Pour remaining marinara sauce over the top

Sprinkle remaining mozzarella cheese evenly over the dish

Cover with foil and bake for 25 minutes, then uncover and bake an additional 10 minutes until bubbly and golden

Garnish with fresh parsley and serve warm