Cook the lentils in a saucepan of simmering water until they are soft but still have some bite. This will take 15-20 minutes depending on the age of the lentils.
Meanwhile pout the vinegar into a small bowl and add the red onion. Stir well then leave the onion to soak to remove some of its pungency.
Once the lentils are cooked drain and rinse with warm water then tip into a serving bowl. Immediately squeeze over the lemon juice and 2 tbsps extra virgin olive oil then stir well. Leave to cool completely.
Drain the red onion, reserving the vinegar for the dressing , and stir it in together with the red peppers, tomatoes, lemon zest, garlic and basil.
Dress the salad by stirring through a tbsp of the reserved vinegar, the remaining 3 tbsps extra virgin olive oil, 1 tsp salt ans 1/2 tsp pepper. Add more lemon juice, vinegar or salt if needed.
Just before serving strew with a few more basil leaves and the feta if using.