Prepare the Vegetables and Sauté: Slice onions and peel, then cube carrots. Sauté in cooking oil over medium-low heat for 5-8 minutes until softened. Add cumin and tomato paste, cooking for 2 minutes.
Simmer the soup: Rinse red lentils until water runs clear. Add to the pot with a chicken stock cube and water. Simmer for 30 minutes until lentils are soft.
Finishing touches: Melt butter in a small pan, stir in chili flakes for 30 seconds, then drizzle over the soup. Squeeze fresh lemon juice before serving.