Make batter: In a blender blend batter ingredients until smooth and transfer to a bowl. Let batter stand, covered, 1 hour.
Preheat oven to 425 degrees.
Put 2 mini-muffin pans (each containing twelve 1 3/4 by 1-inch cups) on 1 baking sheet and spoon 1 teaspoon oil into each cup. Put the baking sheet in middle of oven 3 minutes to heat oil in cups. Working quickly, pour 2 teaspoons batter into hot oil in each cup and bake in middle of oven 18 minutes, or until pudding shells are golden and puffed. Remove shells from cups with tongs and cool on racks. Repeat procedure with remaining batter. Shells may be made 3 days ahead and chilled, covered, or 1 month ahead and frozen in airtight containers. Bring shells to room temperature and recrisp in a 300 degree oven 10 minutes.
In a small bowl stir together sour cream, horseradish, and parsley and transfer to a small pastry bag fitted with 1/8-inch plain tip.
Fold a few beef slices into each shell and pipe 1 teaspoon horseradish cream onto each pudding. Garnish puddings with parsley.
(recipe courtesy of Sara Moulton)