Heat 2 tbsp. butter on your medium pot, add your one chopped onion and saute until tender.
Add 1/2 tbsp. tomato paste to the onions and saute a little more
Add 1 cup of red lentil, 1 chopped potato, 1 chopped carrot to your pot. Pour 4 cups of water on, cover a lid on your pot and cook your soup until the vegetables soften 30-40 minutes.
When the soup is cooked, turn off your heat and stir your soup with your hand blender until it gets very smooth.
Add your 1 tsp. black pepper, 1 tsp. ground red pepper, 1/2 tsp. ground cumin, salt and stir until combined. At this point you can add some more water (about 1/2 - 1 cup) of water if you want the consistency of your soup thinner.
Make your butter sauce in another sauce pan and pour onto your red lentil soup.