In a blender or food processor, combine mayonnaise, onion, vinegar, and sugar. Add Italian seasoning, garlic powder, salt, and pepper. Blend until smooth.
Makes 1-1/2 cups of delicious dressing. Keep refrigerated until time to use. Serve on a big Italian-style salad, including things like romaine and iceberg lettuce, tomatoes, radicchio or purple cabbage, shaved carrots, or whatever your favorite veggies are. Enjoy!