Heat oven to 375°F. Lightly grease a 9x13-inch baking dish.
Bring a large pot of salted water to a boil. Cook spaghetti until just shy of al dente. Drain.
Warm butter and olive oil in a wide skillet over medium-high heat. Sauté mushrooms until browned.
Stir in onion and cook until softened. Add garlic and cook 30 seconds, just until fragrant.
Sprinkle in flour. Stir and cook 1 minute to remove the raw taste.
Whisk in broth, milk, and cream. Add sherry if using. Simmer, stirring, until thick and silky.
Season with salt, pepper, and lemon zest. Stir in 3/4 cup Parmesan and the peas until melted and hot.
Fold in turkey, cooked spaghetti, and parsley. Toss gently to coat everything in the sauce.
Transfer to the baking dish. Top with mozzarella if using.
Mix panko with melted butter and remaining Parmesan. Sprinkle evenly over the top.
Bake 20–25 minutes until bubbling and the crumbs are golden. Rest 5–10 minutes before serving.