Perhaps I am lazy but, the best I have had are sooo simple.
Slice the onion to desired thickness. Separate rings. Drop into a bowl and cover with buttermilk. Let them sit for a bit while you mix the breading.
2 c. ap flour, 1 c. cornmeal, 2 tbl. salt, 2 tsp. fresh ground pepper, other seasoning to your liking up to 1 tbl. (granulated garlic or cajun spice or cayenne pepper:be careful with this one, lemon zest, saffron, Italian herbs, etc.)
Put the breading mixture into a zip bag. Using seive or spider, lift rings from buttermilk and shake off excess. Drop into breading and shake. Remove coated rings using (again a seive or spider) shake off excess then place onto a rack to allow the milk/breading to bond to the onion. About five minutes.
Drop rings into 350 degree oil (again your choice, prefer canola or veg. Peanut imparts a off taste.)
Fry in small batches until golden. Remove and place on rack over sheet pan lined with paper towel.