Preheat oven at 180°C and prepare a rectangular baking dish. I used a 6 x 10 x 2½ in. dish.
Heat oil and melt butter in a pan over medium heat. When butter melts, Saute shallots and garlic.
Tip in the shrimps and cook until pink in color. Sprinkle the flour and stir for 2 minutes then pour milk.
Continue stirring until the sauce thickens. Add the fish and green peas and season with a dash of salt and pepper.
Simmer while carefully stirring to avoid the flour sticking at the bottom of the pan. Do this for 7-8 minutes or until the fish is cooked and sauce is creamy.
Transfer in the baking dish and let it cool a bit, just enough as to not melt the puff pastry.
Take puff pastry from the fridge and roll onto the baking dish. Cut excess pastry on the side if needed. Score the top of the puff pastry using a knife to release steam when baking.
Place inside the preheated oven and bake for 20-25 minutes. On the 18th minute mark, you can cover the top with aluminum foil to avoid burning the pastry.
Remove from oven and serve. Bon appetit!