Wash the lettuce, remove the outer leaves if wilted, spin or pat dry until no moisture remains, then tear into bite-sized pieces. Peel and finely chop the red onion. Wash the apple, cut it in half, remove the core and stem, then cut the flesh into small pieces.
Wash the dill, pat dry, and finely chop. Finely dice the pickle. In a bowl, whisk together the sour cream, yogurt, and lemon juice until smooth. Season with salt and pepper to taste.
Cut the matjes fillets into bite-sized pieces. Divide the lettuce among plates, then top with the onion, apple, matjes, and dill. Drizzle with the dressing and serve immediately.