Put the pan back on the heat, add the olives, tomatoesand fish stock, and bubble up so that it reduces by half. Then put the sea bass back into the sauce, crust upward, for a minute or so to heat through.At the same time, put the artichoke puree back on the heat to warm through. Stir in the butter and, when the puree is hot, spoon it onto your plates and put the fish on top.To the pan in which the fish has been cooked, add the reserved lemon juice, the rest of the olive oil and the parsley, then spoon this mixture around the fish and serve.