Add the stock and the lentils and bring to a boil. Reduce the heat and let simmer partially covered, until the lentils are soft and have broken apart (around 20-25 minutes).
Remove the bay leaves. Decide at this point if you would like to puree the soup or not. I like to leave mine chunky.
Season the soup with lemon zest and lemon juice. Decide if more salt and pepper is needed.
To serve, combine the feta cheese, 1 tsp minced rosemary and fresh cracked pepper in a bowl. Sprinkle over hot bowls of soup and enjoy.