½ cup breadcrumbs, plus a bit extra for sprinkling
2 tspns Worcestershire sauce
1 tblspn chopped dried porcini mushrooms
2 eggs, lightly beaten
1/3 cup wholegrain mustard
3 sheets frozen puff pastry, thawed
8 thin slices prosciutto
7 oz. crème fraiche or sour cream
Preheat oven to 400F. Grease or line a baking tray. Combine beef, onion, garlic, tomato paste, herbs, crumbs, Worcestershire sauce, mushrooms, one egg and 2 tablespoons of the mustard in a bowl. Season with salt and pepper.
Cut 2 pastry sheets in half. Top each half with 2 slices prosciutto, then sprinkle with extra breadcrumbs. Divide beef into 4 portions, roll into thick logs and place onto the pastry sheets. Fold over to seal, then place seam side down on the baking tray. Slice remaining pastry sheet into 1cm wide strips and use to decorate the Wellingtons in a lattice pattern. Brush with remaining egg and bake for 20-30 minutes until pastry is golden. Rest for 5 minutes. Combine the remaining mustard with crème fraiche or sour cream to serve.