Season the lemon sole pieces, to taste, with salt and freshly ground black pepper.
Heat the oil and butter in a frying pan over a medium heat and add the lemon sole pieces, skin-side down. Fry for 3-4 minutes, then carefully turn the lemon sole pieces over and continue to cook until the fish is opaque and golden-brown on both sides. Remove the lemon sole pieces from the pan, set aside on a warm plate, then add the brown shrimps to the pan and heat for a further 1-2 minutes, or until warmed through.
For the salad, whisk the olive oil, white wine vinegar and wholegrain mustard together in a large bowl until well combined. Season, to taste, with salt and freshly ground black pepper. Add the herbs and mix thoroughly.
To serve, spoon the salad onto a serving plate. Top with the lemon sole pieces and spoon over the brown shrimps.