To prepare the very best lentil soup with potatoes, soak the lentils in cold water overnight. Remove the lentils that float to the surface and drain and rinse the rest. Trim and wash the celery and carrot and cut into small pieces.
Peel and finely slice the onion. Peel and chop the garlic. Saute the vegetables, mint and thyme leaves in 4 tbsp of oil in a big saucepan.
Add the lentils and allow the flavours to mingle for 5-6 minutes. Add hot water until the lentils are completely covered and then the tomato concentrate. Cover and cook over a low heat for about an hour. Add the diced potatoes, adjust for salt and pepper and cook for 20 more minutes. Serve the soup warm in individual bowls with a drizzle of P.D.O. Umbria oil.