Pat catfish dry. Season lightly with salt and pepper. Dredge in flour, shaking off excess. Measure thickness of fish at thickest part to estimate cooking time; allow 10 minutes per inch of thickness. Heat butter in a nonstick skillet over moderate heat until it bubbles. Add fish and cook for 3 minutes. Turn fish and continue cooking until fish flakes when tested with a fork. Remove to warm plates. Add lemon juice and parsley to pan; cook 30 seconds, stirring to loosen contents in the pan. Pour over hot fish. Garnish with lemon slices.