Bring a small saucepan of water to the boil,add the eggs,simmer for 8 minutes then transfer the eggs to iced water. Once cool,peel and halve the eggs. Combine the olive oil,mustard,lemon juice and a teaspoon or two of the liquid from the pickled onions in a bowl and whisk. Season with salt and add pepper to taste.
Combine the salad leaves,pickled onions, cornichons,apple and celery in a large bowl,pour the dressing over the top and toss well.
Grill the sourdough on a grill pan until well toasted on both sides,then spread with the butter. Divide the salad between four plates and arrange the toast,ham,cheese,Branston Pickle (or chutney) and eggs next to the salad,then serve.