Preheat oven to 400°F. Spray a 9x13-inch baking dish with cooking spray and set aside.
Cut zucchinis in half lengthwise. Use a small spoon to carefully scoop the flesh out, leaving a firm border along the edges. Place zucchini halves in the prepared baking dish (see note 1) and drizzle 1 tablespoon olive oil over the them. Rub the oil into each zucchini and add a small sprinkle of salt and pepper (the filling is salted, so just a pinch here!). Bake for 5 minutes and remove.
Meanwhile, heat olive oil in a large pan, add onion, and cook 3–5 minutes or until onion begins to become translucent. Add in garlic. Cook, stirring constantly, for 30 seconds. Move onion and garlic to edges and add sausage to the middle of the pan. Cook and stir, breaking up with a wooden spoon into small crumbles, until cooked through and no longer pink, about 5–7 minutes. Drain off any grease if needed.
Add all seasonings (oregano, thyme, Italian seasoning, beef bouillon powder, and salt and pepper to taste), tomato paste, and Worcestershire sauce. Stir, cooking over medium heat, 1 minute or until very fragrant. Add in marinara sauce and bring to a simmer. Simmer 5 minutes.
Spoon meat mixture evenly into zucchini shells; it should fill 8 medium zucchini halves. Add freshly grated cheese evenly on top.
Bake 20–25 minutes or until zucchinis are crisp tender (it should still have a bite to it but not be mushy!) and cheese is melted. If desired, add fresh parsley. Serve immediately.
Notes
Note 1: If all the zucchinis won’t fit into one pan, no worries! Either fit zucchini along the sides of the pan or use another pan, such as an 8x8 or 9x9 pan, to hold the overflow.Storage: Zucchini Boats don’t freeze or thaw well, though the meat filling mixture can be frozen for 2–3 months: Let it cool, then store in a heavy-duty freezer bag or container, allowing expansion space. Thaw in the fridge overnight and reheat on the stove over low heat, 15–20 minutes, stirring occasionally.