Brush the cod fillets with a little oil and season to taste. Cook under a preheated hot grill for 4 minutes on each side until cooked through.
Meanwhile, place the olives, anchovies, capers, garlic and thyme in a food processor and pulse until roughly chopped. Stir in the oil and vinegar.
Transfer the fish to serving plates and spoon the tapenade dressing on top. Serve immediately with a tomato and red onion salad.