Wash the lentils well and leave them to drain. Peel the onions, chop them into medium-sized pieces.
Peel the carrots and potatoes, dice them. Pour a little oil or olive oil in a pot (your choice), add the vegetables.
Saute on the stove about 10 min. Add the lentils. Pour on hot water and leave to boil until softened. Cooking time is roughly 25 min.
Melt 1/2 the butter in another pan and add the tomato paste. Wait for it to come to a boil, then remove from the stove and add the spices - salt, black pepper, spearmint and paprika. You're going to get a thick mixture.
Blend the boiled soup with a hand blender. Add the tomato mixture and stir with a wooden spoon until mixed. If needed, pour in additional water to the blended soup. The goal is to obtain a cream soup of a suitable consistency.
Taste it and if needed, add some more salt and black pepper.
Right before serving, melt the resulting butter in the pan, add the paprika and cayenne pepper. Distribute into the soup with a spoon. Decorate with lemon slices.