Season the fish with cracked pepper. In a small bowl, blend together the lemon juice, zest and 2 tablespoons of butter. Spread over both sides of the fish.
Melt remaining butter in a nonstick frying pan over moderate heat. Add the fish and cook for 2 to 3 minutes on each side, basting until tender and golden, flaking easily with a fork.
Portion the already steamed asparagus onto serving plates, top with the fish and drizzle with the lemon butter in the pan.
