Step 1

Heat the oil in a large frying pan over a medium-high heat, add the shallots and garlic, then add the mushrooms and cook until all the moisture from the mushrooms has evaporated.

Step 2

Add the chopped parsley.

Step 3

Dry the fillet of beef with a paper towel, use a sharp knife to make a crisscross diamond pattern on the surface, and sprinkle with the thyme, plenty of salt, and pepper.

Step 4

In a large frying pan heat the oil over a high heat and sear the beef on all sides and each end.

Step 5

Now spread mustard over the entire fillet, wrap in cling film, and leave to chill in a refrigerator. (This may be done up to 24 hours ahead.)

Step 6

When you are ready to cook the beef preheat the oven to 450°F. Roll out the puff pastry to a size that will completely wrap around the beef plus an extra inch or two for sealing.

Step 7

Spread the duxelles over the pastry.

Step 8

Lay the beef on top, then fold the pastry round it and moisten and seal the pastry edges.

Step 9

Brush the pastry with the beaten egg and use a sharp knife to make a crisscross pattern.

Step 10

Sprinkle the pastry with some coarse sea salt to help it to crisp.

Step 11

Bake for 30 to 35 minutes,  until the pastry is crisp and golden, then remove from the oven and allow to rest for 10 to 12 minutes.

Step 12

Using a  really sharp knife, cut into slices and serve.