Preheat oven to 375 degrees F (190 degrees C). Grease a 9-inch 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 broccoli; cook 3 more minutes.
Whisk heavy cream, eggs, salt, nutmeg, black pepper, and Italian seasoning in a bowl.
Layer broccoli mixture over the puff pastry and top with Brie cheese slices. Pour in egg mixture.
Bake in the preheated oven until set and golden, about 30 minutes.
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