How to Make It
Preheat oven to 400°F. Line a baking sheet with aluminum foil; coat with cooking spray.
Place zucchini between a double layer of paper towels; squeeze to extract excess moisture. Stir together zucchini, turkey, onion, oregano, salt, pepper, and egg in a bowl. Form 24 meatballs with turkey mixture, about 1 tablespoon each; arrange on prepared baking sheet. (Turkey mixture will be very soft, but it firms up as meatballs cook.) Bake at 400°F until fully cooked, about 12 minutes.
Meanwhile, heat a large ovenproof skillet over medium-high. Add oil to pan; swirl to coat. Add mushrooms and garlic; cook, stirring occasionally, until mushroom liquid is mostly evaporated, about 5 minutes. Stir in marinara sauce and 1/4 cup water; reduce heat to medium, and simmer 5 minutes. Add meatballs to skillet (leave behind cooked-out proteins); gently stir to coat with sauce. Sprinkle evenly with cheese.
Preheat broiler with oven rack in top position. Broil meatball mixture just until cheese is melted and bubbly, about 2 minutes. Serve with toasted baguette.