Heat olive oil in heavy large saucepan over medium heat. Add garlic and stir 30 seconds.
Add vegetable broth and lentils; bring to boil. Reduce heat, cover and simmer 10 minutes.
Add potatoes; cook uncovered until potatoes and lentils are tender, stirring occasionally, about 15 minutes.
Grate 1/2 tsp peel from lemon; squeeze enough juice from lemon to measure 2 Tbsp. Add lemon peel, lemon juice, spinach and cayenne to stew.
Cover and simmer stew until spinach wilts and is cooked through, about 2 minutes. Mix in mint. Season to taste with salt and pepper.
Spoon stew into large soup bowls.
Sprinkle feta cheese over, if desired.