Melt the butter in a saucepan and fry the onion, celery and carrots for 3 minutes. Add the lentils, stock, wine, herbs and a little salt and pepper. Bring to the boil, then reduce the heat and simmer gently, uncovered, for about 20 minutes or until the lentils are tender.
Meanwhile, mix together the ingredients for the gremolata.
Heat the oil in a frying pan. Add the mushrooms and fry for about 2 minutes until golden. Season lightly with salt and pepper.
Ladle the lentils on to plates, top with the mushrooms and serve scattered with the gremolata.