Rinse the red lentils under running water until the water runs clear. Dice the onion, carrot and potato evenly.
Heat the olive oil and 1 tablespoon butter over medium heat. Soften the onion for 3 minutes. Add the carrot and potato and sauté for 4 minutes. Sprinkle in the flour and stir for 1 minute.
Add the lentils, cumin, salt, black pepper and hot water. Bring to a boil, skim any foam, then simmer partly covered for 22–25 minutes until completely tender.
Remove from the heat and blend until smooth. Add a little hot water if needed. Return to the heat, simmer for 2 minutes and adjust the salt.
Melt 1 tablespoon butter in a small pan. Stir in the paprika for 10–15 seconds without burning it. Drizzle over the soup and serve with lemon.