Spread 2 Tbsps of the Dijon mustard over the pork fillets and leave to marinate for about 10 minutes.
Meanwhile, slice the leek, garlic and mushrooms.
Heat a little olive oil in a large frying pan over a medium heat. Cook the leek, garlic and mushrooms for about 8 minutes until softened, adding a splash of water if needed.
Stir in the lentils, apple cider vinegar, crumbled stock cube, lemon juice and the remaining Dijon mustard. Add a splash of water, cover and simmer gently over a low heat.
Stir in the chopped kale and cook for 4–5 minutes, or until just tender.
Meanwhile, heat a griddle pan or barbecue until hot. Sear the pork fillets for about 30 seconds on each side.
Reduce the heat and continue cooking, turning the fillets every minute until cooked through. The exact cooking time will depend on their thickness, so take care not to overcook them.
Leave the pork to rest for a few minutes before slicing.
Serve the sliced pork over the lentils, mushrooms and kale with vegetable mash, buckwheat, brown rice or quinoa.