Grind the anchovies and garlic in a pestle and mortar, or chop both very finely.
Stir in the lemon juice and mustard until well combined. Slowly whisk in the olive oil, a little at a time, until you have a rich emulsified dressing. Taste and add some salt if needed.
Put the mixed leaves into a salad bowl, pour over the dressing and toss to combine just before serving.