Cut the pastry into eight squares. Take four squares, cut out a smaller square from inside, remove the smaller piece to form a square frame.
Lay one frame directly on top of one whole pastry square. Gently press to seal the edges. Repeat with the remaining frames and squares.
Brush the pastry squares with egg. Bake in the preheated oven for 25 minutes.
Meanwhile, heat 25g butter in a frying pan. Saute the onions over medium heat until tender.
Add leeks and carrots then saute for an additional 10 minutes. Season with salt and pepper.
Heat the remaining butter in another frying pan. Pan sear the scallops over high heat for 2 minutes per side. Deglaze the frying pan with Cognac and keep warm.
Fill each puff pastry shell with vegetables and 2 scallops.
Transfer back into the oven for 15 minutes. Remove and serve immediately.