To make the marinade, combine the oil, lemon zest and juice, thyme, oregano and pepper in a large bowl.
Add the mushrooms to the marinade and toss to coat well. Leave to marinate at room temperature for about 1 hour.
Meanwhile, rinse the lentils in a sieve under cold running water. Tip into a saucepan and cover with plenty of water. Bring to the boil, then partly cover the pan and simmer for 15–20 minutes or until the lentils are almost tender. Drain well.
Heat a large non-stick frying pan over moderate heat. Add the mushrooms with the marinade and sauté, stirring constantly, until softened and browned.
Add the lentils, tomatoes with their juices, stock and olives. Stir to mix. Bring to the boil, then simmer, partly covered, for 10–15 minutes or until the lentils and mushrooms are very tender. Serve hot or warm.