Prepare the Vegetables: Begin by chopping the onion, leek, carrot, and potato into small, even pieces. This ensures that they cook evenly and blend well into the soup.

Sauté the Aromatics: Heat the olive oil and butter in a large pot over medium heat. Add the chopped onion and leek, and sauté for about 5 minutes, or until they become soft and translucent. The butter adds a rich flavor, while the olive oil keeps the dish light.

Add the Vegetables and Spices: Stir in the chopped carrot, potato, salt, pepper, and a pinch of cumin. The cumin adds a warm, earthy flavor that complements the sweetness of the carrots and the starchiness of the potatoes. Cook for another 5 minutes, stirring occasionally, until the vegetables start to soften.

Incorporate the Lentils and Tomato Paste: Add the red lentils and tomato paste to the pot. Stir well to coat the lentils and vegetables with the tomato paste, which will give the soup a deep, rich color and flavor.

Add the Liquid: Pour in the boiling water or broth. If you prefer a richer soup, use broth instead of water. Bring the mixture to a boil, then reduce the heat to low and let it simmer for about 20 minutes, or until the lentils and vegetables are fully cooked and tender.

Blend the Soup: Once everything is cooked, you can either leave the soup as is for a chunkier texture or use an immersion blender to blend it until smooth. Blending will create a creamy, velvety texture without the need for any cream.

Add the Finishing Touches: Stir in the chopped cilantro or parsley and the lemon juice. The fresh herbs add a burst of freshness, while the lemon juice brightens the flavors and adds a slight tang.

Serve: Ladle the soup into bowls and garnish with a little extra cilantro or parsley if desired. Serve with a wedge of lemon on the side for an extra citrusy kick.