Preparing the hay jus

1. Bring the beef jus to a boil, add the hay and infuse without reducing for about 30 minutes, covered.
2. Strain the jus through a fine strainer, add the wine vinegar, boil for 5 minutes and season.

Beef filets

1. Quickly brown the beef filets in a sauté pan in the duck fat, then wrap individually in cheesecloth.
2. Divide the clay into 4 parts; spread each piece out between 2 sheets of parchment paper to a 5 mm (1/4") thickness.
3. Cut out 4 rectangles the width of the terrine and the rest of its total surface.
4. Line the bottom of 4 small porcelain terrines with parchment paper and then a large sheet of clay.
5. In the center place a small amount of hay and spelt, place the filets on top and cover with hay and spelt.
6. Moisten the bases of the rectangles then close with the clay lid; press down well to seal the two layers.
7. Unmold, smooth the top by hand, using some water if necessary, and form a little chimney in the top of the clay.
8. Heat the oven to 175° C (350° F). Place the clay parcels directly onto the rack to bake. Cook for 10-20 minutes, then check the temperature in the center of the fillets by inserting a meat thermometer through the chimney.
9. Remove immediately and open up the clay.

Garnish

1. Cook the vegetables in boiling salted water leaving them crisp.
2. Finish cooking in a sauté pan with a knob of butter and a little white chicken stock; season.

Finishing and presentation

1. Arrange the vegetables in a wreath on the plates; in the center place the mustard or rocket leaves dressed with a drizzle of walnut oil.
2. When the beef is cooked, cut in half and place on the leaves, spooning around the hay jus.