Attach a candy thermometer to a Dutch oven or other large heavy pot. Pour 3 inches of oil and set the pot over medium-high heat. Heat the oil to 350-375 degrees.
While the oil is heating, beat the egg, cream and water together in a shallow dish.
Put the whey powder and seasoning salt in a gallon-sized resealable plastic bag. Seal and gently shake
Dip onion rings in the egg mixture a few at a time, allowing the excess to drip back into the bowl, then place them in the bag with the whey protein. Seal the bag and shake gently to coat the onion rings. Repeat until all onion rings are coated.
Working in small batches, fry the onion rings until golden brown on each side. Drain on a paper towel lined plate. Add more salt to taste. Best served immediately.