Melt butter in a skillet over low heat; add onion, and cook, stirring occasionally, 45 minutes or until caramelized.
Combine whipping cream and next 2 ingredients, stirring with a wire whisk until sugar dissolves and mixture is smooth. Add salt and caramelized onion to custard mixture. Place mixture in container of an electric blender, and process until smooth. Pour mixture evenly into 5 (5- x 1-inch) round individual baking dishes; place dishes in a large roasting pan or a 15- x 10- x 1-inch jellyroll pan. Prepare 1/2-inch water bath (see Brûlée Basics below).
Bake at 275° for 50 minutes or until almost set. Cool custards in water in pan on a wire rack. Remove from pan before serving.
Note: Baking time is for 5- x 1-inch round individual baking dishes. As a general rule, to use 4-, 6-, or 8-ounce custard cups, bake for an additional 15 to 20 minutes. When the crème brûlée is done, the center will still be slightly liquid and a knife will not come out clean. The yield will vary with different size dishes: For 4-ounce cups you'll get 10 servings, for 6-ounce cups you'll get 7 servings, and for 8-ounce cups you'll get 4 servings.
Don't panic when you see the term water bath. A water bath is simply a roasting pan or jellyroll pan filled with water. The water creates a cushion from the heat of the oven, allowing the custards to bake slowly without curdling.
Don't burn yourself. Before you take the water bath out of the oven, remove some of the water with a basting bulb or a long-handled ladle.