Cut off the stalk of the artichoke and remove any hard outer leaves. Rub with half a lemon.
Squeeze the remaining lemon juice into a pan of rapidly boiling, salted water. Plunge the artichoke into the water and boil uncovered for 25 to 30 minutes, until the outer leaves are easily detached. Drain well.
Turn the artichoke upside down and squeeze out the water.
While you are cooking the artichoke, make the vinaigrette. Combine the vinegar and mustard in a small lidded-jar and season to taste.
Pour in the olive oil, before screwing on the lid and shaking vigorously to make a thick dressing. Taste, adding more mustard, vinegar and seasoning if necessary. The dressing will keep in the fridge for several days
Remove the artichoke chokes carefully using a spoon, and serve straight away with the vinaigrette, before they discolour.