Heat 1" of oil on high in a heavy bottom pan (I use a skillet). I like to use a smaller pan for this so I don't need as much oil. Alternatively, you can use a deep fryer set to 365 F.
Separate large onion (1) slices into rings and set aside.
In a wide-mouthed bowl, combine whey protein (1 cup), xanthan gum (2 teaspoon), salt (1 1/2 teaspoon), baking powder (2 teaspoon), and grated parmesan (1/2 cup).
Add soda water (6 tablespoons) and whisked egg (1), and stir until well combined. The combo will be wet.
Once the oil is hot (about 375 F), use tongs to transfer the onion rings into the coating, then directly in the oil.
Allow the onion rings to fry until golden, then transfer them to a paper towel-lined plate. Continue until you've fried all the onions rings.
Serve immediately (will get soggy over time) and enjoy!
Notes
Frying Oil should have a high smoke point and mild flavor. Please keep in mind that all oils are low in carbs, but some are considered "healthier" based on polyunsaturated fats. If this is a concern to you, opt for avocado oil, or peanut oil (slightly higher). If not, vegetable or canola are great frying oils. I use peanut oil for frying.
Xanthan Gum is necessary for binding the ingredients of this recipe together, you CAN'T SKIP OR SUB!