Preheat an oven to 450 degrees F (230 degrees C). Spray a 9x13-inch casserole dish with cooking spray.
Evenly spread spinach in the bottom of prepared casserole dish; sprinkle with garlic salt. Arrange fish atop spinach, keeping the pieces side by side and larger pieces toward the outside of dish. Top fish with lemon zest, lemon juice, dill, sea salt, and black pepper.
Combine Cheddar cheese, Italian-seasoned bread crumbs, and panko bread crumbs in a bowl; spread evenly over fish. Cover dish with aluminum foil.
Bake in the preheated for 12 to 15 minutes; uncover and cook until fish flakes easily with a fork and topping is lightly browned and crunchy, 5 to 8 more minutes.
