Scrub the beets clean under running water. Fill the bottom of your pot with water, place the steamer inside and add the beets.
Cover the pot and bring the water to a boil. Reduce heat to a simmer and steam the beets for 40 minutes, or until a knife pierces them easily. Remember to top up the pot with water, if necessary.
Remove the beets from the pot and set aside until cool enough to handle. Top and tail, then rub the skin off with some kitchen towel. Cut into cubes.
Heat the lentils in the microwave according to packet instructions and then transfer to a large bowl. Add the cubed beets and feta cheese and drizzle with oil and vinegar.
Season with salt and pepper and sprinkle with the chopped mint before serving.