In a 1- to 1 1/2-quart non-reactive saucepan, mix sugar and cornstarch. Stir in vinegar. Stir over high heat until mixture boils and thickens, about 2 minutes. Pour into a blender and add onion, garlic, ketchup, Worcestershire, and cayenne; blend on high until smooth. With blender still running, gradually pour in oil. Stir in celery seed. Add salt to taste. Let cool. Serve over fresh salad greens. If making ahead, cool, cover and chill in refrigerator for up to 1 week. Makes about 1 3/4 cups.