Cook the lentils in a large saucepan of salted boiling water for 35–40 minutes, or according to the packet instructions, until cooked al dente. Drain and transfer to a large salad bowl.
Heat the olive oil in a frying pan and add the onion and red pepper. Cook over a medium heat for 15 minutes until beginning to soften, then add the garlic and cumin seeds and cook for 1 minute. Add the mushrooms, season with salt and pepper and cook for 2–3 minutes until they are softened.
Add the cooked vegetables to the warm lentils, stir in the lemon rind and juice and the parsley, and drizzle well with the oil. Season to taste. Allow the salad to cool to room temperature, then add the leftover turkey. Leave to marinate for at least 30 minutes before eating.