Place the lentils into a pot and cover completely with cold water until it reaches about 1 inch from below the top edge of the pot. Place over high heat, bring to a boil and reduce to medium-low heat. Continue cooking the lentils on a gentle simmer until just tender, about 20 to 30 minutes. The lentils should still have a bit of a bite to them. Rinse and drain the lentils. 

Mix the lemon juice, olive oil, salt and pepper in a mixing bowl until combined. Add the cooked lentils into the olive oil and lemon juice mixture. Mix until all the lentils are coated. Cover the bowl with plastic wrap and set aside in the refrigerator for about one hour. 

Transfer the lentils from the refrigerator. Add the cucumbers, onions, feta and parsley into the lentils. Mix well until all the ingredients are distributed evenly and coated with the olive oil and lemon dressing.

Serve cold.