Dilute the flour with a little water to a sludge.
Heat the oil in a suitable container and put the floury sludge in it. Once thickened a bit, pour in a little broth.
Once the sauce thickens again, pour in a little more broth - repeat this 2-3 more times until you've added all the broth.
When thickened for the final time, add the egg yolk beaten with lemon juice. Stir.
Leave to simmer a few more min. and remove from the heat. Sprinkle with fresh parsley.