Cook the lentils in a large pot of water with the chicken broth for about 20 minutes. They should still be a little crunchy.
Once cooked, drain and run under cold water to cool.
Wash the cucumber and cut it into cubes. Cut the tomatoes into cubes. Finely chop the mint.
Put the lentils, cucumber, tomatoes, feta cheese and mint in a salad bowl.
Season with lemon juice, salt and olive oil.