Preheat your oven to 375 degrees Fahrenheit. Wash the zucchini thoroughly, trim off the stem ends, and slice each zucchini in half lengthwise. Using a spoon or melon baller, scoop out the center flesh, leaving a 1/4 to 1/2 inch rim. Chop the scooped-out zucchini flesh into small pieces and set aside.

Arrange the hollowed zucchini halves on a parchment-lined baking sheet. Brush the insides with 1/2 tablespoon of olive oil and season with 1/4 teaspoon of salt and 1/4 teaspoon of black pepper. Pre-bake for 15 minutes, then remove from the oven and set aside.

While the zucchini bakes, heat a large skillet over medium heat with the remaining 1 tablespoon of olive oil. Add the Italian sausage and cook until browned, breaking it apart with a spatula.

Once the sausage is browned, add the diced onion and minced garlic to the skillet. Continue sautéing over medium heat until the onion becomes translucent and the garlic is fragrant.

Stir in the Italian seasoning, the remaining 1/2 teaspoon of salt, 1/4 teaspoon of black pepper, garlic powder, marinara sauce, breadcrumbs, and half of the reserved chopped zucchini flesh. Mix well and let the filling simmer over medium-low heat for 5 minutes.

Stuff the pre-baked zucchini boats with the sausage mixture, dividing it evenly. Top each boat with shredded mozzarella cheese. Return to the oven and bake for another 15 minutes, until the cheese is melted and the filling is bubbly around the edges. Garnish with fresh chopped parsley and serve warm.