Thinly slice the leek and the mushrooms. Cut the tomatoes in half.
Heat a spoonful of oil in a pan then add the leek. Cook for 5 minutes, then add the mushrooms, chilli flakes and thyme leaves. Cook for another 5 minutes, then add the tomatoes, garlic and a dash of balsamic vinegar.
Cover and cook for 10-15 minutes, until the mushrooms and tomatoes have released their juices.
Drain the lentils and add to the stew. Season with salt and pepper.
Warm through, then add the chopped herbs.
Serve with a dash of balsamic vinegar.