Heat the butter in a frying pan until it has melted and is beginning to froth. Add the shallots and fry gently for 5—6 minutes until softened. Add the wholegrain and Dijon mustards and the tapenade and pour in the cider. Simmer gently for 2 minutes, then remove from the heat.
Press the fillet steaks into the black pepper. Cook on a hot barbecue for 3—4 minutes on each side or until cooked to your liking. Rest on warm serving plates while finishing off the tapenade.
Stir the crème fraîche and tarragon into the tapenade, season to taste with salt and pepper and gently warm the sauce without letting it boil. Pour a little of the sauce over each fillet steak and serve immediately with steamed asparagus, homemade chips and a dish of mustardy mayonnaise.