Whisk egg yolk, apple cider vinegar, mustard & salt until smooth
Very slowly drizzle in the olive oil while whisking continuously (alternatively use a food processor)
Start slowly until the emulsion thickens, then increase to a steady stream
Once thick & glossy, fold through parsley & lemon zest
Fold in finely sliced radishes
Taste & adjust salt or acidity if needed with lemon juice or more vinegar
