Preheat the oven to 375 degrees F (190 degrees C). Grease a 9-inch pie pan with cooking spray.
Melt butter in a saucepan over medium heat. Add onion and cook until tender, about 5 minutes. Add garlic and cook until fragrant, about 1 minute. Mix in broccoli and cook until starting to soften, about 3 minutes. Remove from the heat.
Whisk cream, eggs, salt, nutmeg, pepper, and Italian seasoning together in a bowl.
Line the prepared pie pan with puff pastry, covering the base and sides. Trim excess pastry. Brush mustard over the bottom. Layer with broccoli mixture and Brie slices, then pour egg mixture over top.
Bake in the preheated oven until set in the middle and golden on top, about 30 minutes.