Season the salmon steaks with salt and pepper and dot with butter. Place them on a greased piece of foil on a grill rack and cook under a preheated moderate grill for 10-12 minutes until the fish is thoroughly cooked, turning them over halfway through cooking.
Meanwhile, mix the pesto sauce with the crème fraîche. Pour the mixture into a small saucepan and heat gently for 2—3 minutes until cooked through. Season to taste.
Arrange the salmon steaks on warmed plates and serve the sauce separately. Serve with griddled green vegetables and garnish with basil leaves and black olives.