Cut the zucchini in half lengthwise and trim off the ends. Then carefully scoop out the seeds of the zucchinis with a spoon.
Place the zucchinis in a 9X13 baking pan with the skins on the bottom of the pan. Drizzle the olive oil over the zucchinis. Bake for 12-15 minutes until the zucchini is soft.
While the zucchini is baking, prepare the meat mixture.
Brown the sausage and the onion in a large pan over medium high heat. Break up the sausage with a spatula and cook for 4-5 minutes until all of the sausage has been browned. Drain off the excess grease.
Then add in the minced garlic and cook for 1 minute.
Stir in the crushed tomatoes, Italian seasoning, salt and pepper.
Then spoon the meat mixture into the zucchini shells and top with the shredded mozzarella cheese.
Bake for an additional 5 minutes to melt the cheese.