Stir together the crème fraîche, mayonnaise, dill, the chopped cornichons (or gherkin), lemon juice, Dijon mustard and onion powder (if using).
Season to taste with salt and pepper and keep the dressing in the fridge until you are ready to serve.
Salad
Cook the pasta according to the packet instructions, then rinse it under cold water. Leave it to drain in a sieve.
Rinse all the vegetables thoroughly and cut them up: tear the lettuce into large pieces, halve the cucumber lengthways and cut it into slices, dice the avocado, halve the tomatoes and slice the onion into rings.
Slice the eggs with an egg slicer.
Assemble the salads with a good handful of lettuce in the bottom of each bowl, then build up with pasta, cucumber, tomatoes, avocado, tuna, egg and red onion.
Season with pepper and top the salad with the dill dressing.