In large saucepan over medium-low heat, sauté garlic and ginger in olive oil for 2 to 3 minutes. Add onion and celery and continue to sauté for a further 5 minutes, until onions are translucent. Add celery, carrots, sweet smoked paprika, and 1/4 teaspoon salt. Stir and continue to sauté for 2 to 3 minutes.
Next, add potatoes and lentil, stirring well to coat, and 4 cups of stock. Simmer, increasing heat to medium if necessary, for approximately 20 minutes, until lentils and potatoes are soft. Add spinach, remaining 1/4 teaspoon salt, and remaining 2 cups stock. Cover with lid and cook for 5 minutes or until spinach is just wilted.
Spoon into bowls and sprinkle lemon zest over each serving. Serve wedges on the side and squeeze into soup before eating.