Preheat oven to 400°F (200°C). Line a baking sheet with parchment paper.

Melt butter in a medium skillet over medium heat. Sauté onions, garlic, carrots, and celery until softened.

Sprinkle flour over vegetables and cook for 1–2 minutes to form a roux.

Slowly whisk in heavy cream and seafood stock. Add thyme, salt, pepper, and lemon juice. Simmer until thickened.

Fold in lobster meat and peas, gently combining without overcooking.

Pour filling into greased ovenproof dish.

Cover with puff pastry, trim edges, cut slits, and brush with beaten egg.

Bake 25–30 minutes until golden brown. Let rest 5 minutes before serving.