Step 1In a food processor or blender, blend the egg and lemon juice for 10 seconds. With the processor running, slowly pour in the oil and process until the mixture thickens. Add the pepper and salt and pulse once or twice to blend. Transfer to an airtight container and refrigerate for at least 30 minutes before using (best if used within 24 hours). This makes 1 cup mayonnaise.
Step 1Bring a large pot of salted water to a boil. Add the diced potatoes and bring to a simmer (it should not come to a rapid boil) and cook until the potatoes are slightly tender, about 6 to 8 minutes. Drain the potatoes, put them on a sheet pan and place the pan in the refrigerator to dry.
Step 2Mix together the mayonnaise, creme fraiche, celery, mustard, lemon juice, chopped pepper and dill. Fold in the potatoes and season to taste with the salt and white pepper.