Step 1 Roast the bell peppers over an open flame or in the broiler, turning until blackened all over. Place them in a paper bag inside a plastic bag and tie it closed; leave the peppers to steam in the bag, about 20 minutes. Remove the blackened skin and the seeds and cores.
Step 2 Place the peppers in a blender and puree until almost smooth. Add the garlic, serrano chile, egg yolk and season with one-fourth teaspoon of salt and one-fourth teaspoon of pepper. Puree again until smooth.
Step 3With the blender running, add the olive oil in a slow drizzle, mixing until emulsified. Taste and adjust the seasoning as desired. This makes about 2 cups of rouille. Cover and refrigerate up to 1 day if not using immediately.