Cook shrimp, uncovered, in boiling water about 2 minutes or until opaque; drain. If using pea pods, in a small amount of lightly salted, boiling water cook the pea pods, covered, for 2 to 3 minutes or until tender. Drain, rinse, and cool. Wrap a pea pod around each shrimp. Secure with a toothpick. Cover and chill for 2 to 24 hours.
For Mustard Cream Sauce, in a small saucepan melt butter or margarine. Stir in the flour. Add the half-and-half or light cream all at once. Cook and stir over medium heat until thickened and bubbly. Reduce heat; cook and stir for 1 minute more. Remove from heat.
Stir sour cream, vinegar, mustard, and pepper into cream mixture. Pour into a small serving bowl; cover and chill for 2 to 24 hours.
To serve, sprinkle sauce with capers. If desired, line a platter with romaine leaves; arrange shrimp on leaves. Makes 12 servings.