In a heavy Dutch oven or stockpot, melt butter over moderate heat and saute the onions, half of the piquillo peppers, and salt and pepper until the onions turn a light golden color, 5 to 7 minutes. Stir in the garlic and cook briefly just to release the aroma, 1 to 2 minutes.
Pour in the chicken stock, add the potatoes and bring to a boil. Reduce to a simmer and cook 20 minutes. Puree in a food processor or blender just until smooth, being careful not to overprocess. Overworking will result in a gummy soup. Return to the pot, stir in the crema or creme fraiche or sour cream and bring just to a boil. Julienne half of the remaining piquillos and stir into the soup.
Puree the remaining piquillos in a mini processor and put in a bowl. Whip the cream until soft peaks form and gently fold in the pureed piquillos and the cheese.
Divide hot soup into bowls, top each with a dollop of the piquillo cream and serve.
Whisk the cream and buttermilk together. Cover and set in a warm place (a gas oven with just the heat from the pilot light is fine) for 8 hours. Crema may be kept in the refrigerator as long as a week.