Preheat the oven: Preheat your oven to 375°F so it's ready as soon as the skillet mixture is assembled.
Salt the zucchini: Toss the shredded zucchini with 1/2 teaspoon salt in a colander and let it sit for 10 minutes to pull out excess water.
Squeeze zucchini dry: Transfer the zucchini to a clean kitchen towel and squeeze firmly to remove as much liquid as possible; set aside.
Brown the turkey: Heat olive oil in an oven-safe skillet over medium heat, add the onion, and cook 3 minutes until softened. Add ground turkey and garlic, breaking the meat into small crumbles, and cook 6 to 8 minutes until no pink remains.
Season the mixture: Stir in Italian seasoning, salt, and pepper, and cook 1 more minute until fragrant.
Add zucchini and sauce: Fold the drained zucchini and marinara sauce into the turkey mixture and stir until evenly combined.
Stir in the egg: Remove the skillet from heat briefly and stir in the beaten egg so it binds the mixture without scrambling.
Spread it flat: Press the mixture into an even layer across the bottom of the skillet with a spatula.
Top with cheese: Scatter mozzarella evenly over the top, then sprinkle with Parmesan cheese.
Bake until bubbly: Bake at 375°F for 20 to 25 minutes, until the cheese is melted and the edges turn golden and start to pull from the pan.
Rest and slice: Let the casserole rest 5 minutes before slicing so it holds together when served.