In a small saucepan, boil the orange juice and shallot over medium-high heat until reduced by half, 2 to 3 minutes. Remove from the heat and stir in the orange peel, lemon juice and salt. Let stand for 3 minutes, then strain the mixture over a bowl, pressing to extract any liquid. Return the strained liquid to the saucepan, place over low heat and stir in the butter, 1 tablespoon at a time, until incorporated. Cover the sauce to keep warm.