Step 1Combine the garlic and cayenne pepper in a large mortar and mash with a pestle.
Step 2Dunk the bread into the hot fish soup, squeeze out the soup, add bread to the mashed garlic and mash with the pestle.
Step 3Add the mashed potatoes and mash the mixture into a thick paste.
Step 4Add the olive oil drop by drop, never ceasing to mash the mixture with the pestle in a circular motion until the rouille is thick. (If the mixture breaks up, mash in a teaspoon or two of hot fish soup or a little more soup-soaked bread.) Season with salt and pepper.