Place lentils in a large pot with enough water to cover plus about an inch or two beyond that. Bring to a boil, and cook until lentils are tender, about 15-20 minutes. Drain and rinse with cold water in a colander. Let stand for a few minutes to drain well and cool off a bit. Finely chop all veggies, put in bowl and add Feta. In a separate bowl whisk olive oil, lemon juice, garlic, salt and pepper. Once lentils have cooled a bit (they don't need to be completely cold) add them to the bowl with the veggies. Pour in olive oil/lemon juice mixture and mix well. Allow to sit in fridge a few hours at least before serving.