Preheat oven to 400 degrees. In a bowl, add potatoes and onions. Spray with olive oil and mix potatoes and onions so they are coated with oil. Sprinkle the ranch dressing mix over the potatoes and onion, continue to toss so it coats them.

Spread potatoes and onions in a single layer on greased baking sheet and cook for 20-25 minutes, turning potatoes and then cook another 15 minutes or until potatoes are tender and lightly browned on the top. They outside should be crispy, center tender. Some ovens cook faster, so be sure and check them.

Once removed from oven, let cool. Once cooled add the remaining ingredients and mix well. You may need more mayonnaise as the potatoes will soak it up quickly.