Arrange the salad leaves on a serving plate.
Add the sliced cucumber, red onion, and chopped parsley over the leaves.
Place the drained sardines on top of the salad.
In a small bowl, mix the capers, lemon juice, olive oil, and mustard until combined.
Drizzle the dressing over the salad.
Serve the Summer Sardine Salad right away for the freshest flavor.