Finely slice the leek, garlic and mushrooms.
Heat a Tbsp of olive oil in a large frying pan. Add the leek & chilli flakes, then gently sauté over medium heat for about 5 minutes. Add the garlic, oregano and mushrooms and cook for a further 10 minutes, under cover, until the mushrooms have withered and released their liquid.
Add the lentils and stock, bring to the boil and cook for 10-15 minutes, until the vegetables are soft and the sauce has reduced.
Stir in the lemon zest and juice, then the mustard.
Stir in the mint or parsley, season to taste, then serve immediately with a dash of lemon juice and a pinch of paprika.