Heat oil in a soup pot over medium heat. Cook onion and carrot for 7 minutes, then stir in garlic, tomato paste and cumin for 1 minute.
Add lentils, broth and salt. Bring to a gentle boil, lower the heat and simmer partly covered for 20 to 25 minutes until lentils and carrot are very soft, stirring occasionally.
Remove from the heat and blend with an immersion blender until smooth. Return to low heat if the soup needs warming.
Melt butter in a small saucepan over low heat. Stir in dried mint and paprika for 20 seconds, then immediately stir the seasoned butter through the soup.
Remove the soup from the heat, stir in lemon juice and divide among 6 bowls.