Put the slices of bread on a baking sheet, place in a preheated oven, fry for about 1 minute on each side, until golden brown.
Meanwhile, in a large bowl, combine the mustard powder, lemon juice, black pepper, and olive oil. Put all the salad leaves in this dressing, mix gently.
Break the sardines into pieces and place on toast. Sprinkle capers over the sardines and top with lettuce leaves. Serve the sandwiches right away.