Bring 3 cups water to the boil. Add lentils, thyme and peppercorns. Reduce the heat to low, cover, and simmer for 25 minutes, until lentils are tender. Drain and let cool slightly.
Whisk the lemon juice, oil, garlic and pepper in a large bowl. Add the warm lentils and toss gently to mix. Add the spring onions, roasted red capsicums, fetta and dill. Toss again. To serve, mound lentil mixture on a bed of rocket or watercress.
Common brown lentils are fine for this recipe, but small green lentils from France (often known as Le Puy lentils) are preferred for salads because they retain their shape and have an appealing chewy texture. Total cooking time is about 20 minutes for green lentils and 25 to 30 minutes for brown lentils.