Pour oil into a large stock pot fitted with a thermometer and heat over medium to medium high heat.
Slice onion on the thinnest setting on a mandolin.
Combine buttermilk and milk in a large measuring cup and stir.
In a bowl, combine all dry ingredients and use a fork to combine.
Slowly add wet ingredients while stirring. Let sit for 5 minutes while the oil heats to 350 F in a large stock pot.
Fit a sheet tray with a wire rack and set it beside the stock pot.
Add onions to the batter and use your hands to make sure all the rings are coated.
When the oil has reached 350 F, begin adding the onion rings one at a time, spacing them apart in the oil so they don’t clump. (Don’t crowd the oil because this will cause the oil to lose heat, which will force the batter to take on too much oil and render a very oily ring.)
Flip each ring when the underside is golden brown, then remove from the oil to the wire rack when fully cooked.
Sprinkle with garlic salt.
Repeat until all the onions have been fried.