Heat olive oil in a skillet and sauté onions and garlic until fragrant.
Add chicken pieces, salt, and pepper; cook until lightly browned.
Stir in mushrooms and cook until softened, then add butter.
Sprinkle in flour, cook for a minute, and slowly add stock and milk.
Mix in thyme, Dijon mustard, and parmesan; simmer until thick.
Spoon the creamy filling into a baking dish.
Top with puff pastry rectangles, brush with egg wash, and sprinkle sesame seeds.
Bake at 200°C (400°F) for 25–30 minutes or until golden brown.