Place puff pastry slices next to each other and defrost at room temperature. Wash the meat and dab dry. Hammer thin tips inwards. Heat oil and fry fillets all around.
Take out, season with salt and pepper and let it cool down a little. Mix mustard and chutney and coat meat with it. Whisk the egg. Put 4 slices of dough on top of each other. Roll out each rectangular (approx. 24 x 28 cm) on a floured work surface.
Brush the edges of the dough with egg. Place the fillets on top and wrap them in the dough. Place the packet on a baking tray lined with baking paper with the seam facing down. Coat the fillet packs with egg.
Bake in the preheated oven (electric cooker: 200°C/ convection oven: 175°C/ gas: level 3) for 15-20 minutes. In the meantime, place the Brussels sprouts in 500 ml of boiling salted water without defrosting them. Bring to the boil and continue cooking over medium heat for about 10 minutes.
Arrange sprouts and meat on a plate.