In a blender, put 1/2 cup onion, 1/4 cup vinegar, 3/4 cup oil, 1/2 cup sugar, 1/2 teaspoon salt, dash of celery seed and 3/4 cup ketchup. Blend until smooth. I use sugar substitute.

For an oriental flavor, instead of celery seeds, use 1/4 cup soy sauce, 3 cloves garlic and 1 tablespoon sesame oil.