In a 1- to 1 1/2-quart pan, mix 1/3 cup sugar and 1 teaspoon cornstarch. Stir in 1/2 cup cider vinegar. Stir over high heat until mixture boils and thickens, about 2 minutes. Pour into a blender and add 1/3 cup chopped onion, 1 peeled clove garlic, 1/4 cup catsup, 2 teaspoons Worcestershire, and 1/8 teaspoon cayenne; whirl until smooth. With blender running, gradually pour in 2/3 cup salad oil. Stir in 1/4 teaspoon celery seed. Add salt to taste. Let cool.