Heat oil in a large saucepan over medium heat. Add onion and garlic. Cook for 2 to 3 minutes, or until soft. Add lentils, potatoes and stock. Bring to the boil. Use a slotted spoon to skim any residue from surface of soup.
Reduce heat to medium-low. Simmer, covered, for 25 minutes or until lentils are tender. Remove from heat. Add spinach, 1/2 cup of lemon juice, and salt and pepper. Stir until well combined. Ladle into bowls. Top with lemon rind. Serve with bread.