1. Bring plenty of salted water to a boil in a large pot. Add the lentils and cook over low heat, covered, for about 14min. Drain the lentils, then place them in a bowl.
2. Finely grate the lemon zest, yellow part only (save the zest – approx. 2 tsp).
3. Remove the rest of the peel and the membranes from the lemon – follow the shape of the fruit.
4. Cut along the membranes between the lemon fillets to remove the lemon flesh. Chop the lemon fillets into small pieces and stir into the lentils.
5. Heat 1 tbsp of the oil in a frying pan. Saute the red onions over medium heat, stirring continuously, for about 1min.
6. Add the spring onions and the asparagus, and saute for another 2 minutes, then add the salt.
7. Stir the onions, asparagus and the last tablespoon of oil into the lentils and pour the salad into a serving bowl.
8. Break the feta block up into little rough pieces and press them down between the lentils.
9. Sprinkle with lemon zest and serve with toasted bread rubbed lightly with garlic.