This is a lovely, spicy, warm flavored sauce that can be adapted by adding other ingredients. I often stir in hard-boiled eggs or bread crumbs soaked in vinegar, which adds different textures.
Serve with fish, cooked meats, cheese and salads.
Put the mustard leaves and garlic into a food processor and blend with 2 tsp vinegar. With the processor still running, add the olive oil in a slow stream, season to taste, then add the remaining vinegar if required.
Jekka McVicar is a British horticulturalist who has been called "The Queen of Herbs," and in 2016 the Royal Horticultural Society named her an Ambassador for health and well-being through gardening. Her herb nursery, Jekka's Herb Farm, has won 59 RHS gold medals. She is author of The Complete Herb Book, Jekka's Herb Cookbook and co-author of Vegetables, Herbs and Fruit.