Beef: Arrange the toast points on a work surface, and using the tip of a spoon or a small pastry bag, place a dot (about 1/2 teaspoon) of the horseradish mayonnaise on each. Slice the tenderloin very thin and arrange it attractively on the toast points as well. Garnish each canape with the red onion marmalade and minced parsley. Serve chilled or at room temperature.
Marmalade: In a small skillet combine the sliced onion, red wine vinegar, sugar, two ounces of the water, salt, and pepper. Bring the pan to a boil, and then lower it to a simmer. Cook the onions for about five minutes, stirring often. Dissolve the cornstarch in the remaining 1 ounce of cold water and stir it into the onion mixture. Cook the marmalade for another two minutes. Transfer it to a glass bowl and refrigerate until chilled.