Add remaining 1 tablespoon oil to skillet and stir in asparagus and bell pepper. Sauté 1 minute. Add broth and heat to boiling. Boil uncovered 2 minutes. Stir in lemon peel and lemon juice. When liquid returns to boiling, remove from heat. Beat in butter, 1 piece at a time, with wire whisk, adding the next piece only after the first has been completely beaten in and melted. When all of the butter has been beaten in, stir in parsley.