1. Chop the leek and saute over medium heat with a drizzle of olive oil; Stir occasionally.
2. Add the spices and the potatoes cut into very small cubes and stir.
3. Cover with water and cook for 15 minutes.
4. Remove from heat and wait another 15 minutes for the temperature to drop a little.
5. Transfer the mixture to the blender and add the yeast and broth.
6. Blend well until you get a creamy, lump-free consistency.
7. Transfer the resulting sauce to a plate, and add a drizzle of olive oil and walnuts to finish.