Preheat oven to 190 degrees C. Grease a 23cm pie dish with cooking spray.
Line the prepared pie dish with puff pastry, covering base and sides. Trim excess pastry. Brush bottom with Dijon mustard.
Melt butter in a saucepan over medium heat. Add chopped onions and cook until tender, about 5 minutes. Add garlic and cook 1 minute more. Mix in spinach; cook 1 minute more.
Whisk double cream, eggs, salt, nutmeg, black pepper, and Italian seasoning together in a bowl.
Layer 1/2 of the Gouda cheese over the puff pastry and top with spinach mixture. Pour in egg mixture. Sprinkle the remaining Gouda cheese on top.
Bake in the preheated oven until set and golden, about 25 minutes.