In a small dish, stir together 1 tablespoon of extra virgin olive oil and 1 tablespoon of white wine vinegar, add the slices of shallot or red onion, and cover with the dressing and set aside.
To a small bowl, add the sour cream and 2 tablespoons of fresh dill sprigs and stir together so they’re well combined. Spoon the dill sour cream onto a serving plate and spread it around with the back of the spoon to serve as a base for the sardine salad.
To a big salad bowl, add 2 tablespoons extra virgin olive oil, 1 tablespoon of wholegrain mustard, 1 tablespoon lemon juice, ½ tsp garlic salt, ½ tsp ground cracked black pepper, 1 tablespoon of fresh dill sprigs, and 3 teaspoons of capers, and stir vigorously together to combine well.
To the same bowl, add the capers, cocktail gherkins halves, celery slices, baby cos lettuce leaves, and half the sardines. Stir carefully to combine everything so the sardines don’t break up.
Pluck out the cos lettuce leaves and spread them out on the dill sour cream on the serving plate, then scoop up the sardine salad and spread that over the salad leaves. Arrange the rest of the whole sardines on top of the salad, drizzle the remaining dressing on top, and sprinkle on the last of the dill sprigs.
Serve immediately with warm sourdough toast and any leftover dressing and fresh herbs.