Healthier beef wellington

Name: Healthier beef wellington
Ingredients: 1kg thick, lean fillet of beef; 3 tbsp rapeseed oil; good handful dried porcini mushrooms; 2 shallots, finely chopped; 2 garlic cloves, finely chopped; 140g chestnut mushroom, very finely chopped; 2 tbsp finely chopped flat-leaf parsley; 1 tbsp finely chopped tarragon; 100g mix of watercress, baby spinach and rocket (or use just watercress or spinach); 6 filo pastry sheets, each about 38 x 30cm; 1 tsp plain flour; 5 tbsp red wine; 350ml chicken stock; 2 tsp Dijon mustard