Clean and peel all the vegetables. Cut the potatoes and carrots into pieces and cut the onions into slices. Put the butter in a high-sided pan and place on a low flame. Add all the vegetables and brown them for about 10 minutes. Add the hot vegetable stock and cook for another 30 minutes. Blend everything with an immersion blender and add salt to taste. Brown the bacon in a pan without adding any oil or grease. Drain it on absorbent paper towel and roughly chop it. Pour the soup into individual plates, garnish with the chopped bacon and a bit of oil. Complete with flakes of strong Provolone Valpadana P.D.O., sprinkle with black pepper and serve.