Cut onion rings into ¼-½ inch slices.
Pour buttermilk into a large container and add the onion rings. Let the onions soak in refrigerator for an hour.
Following the fryer manufacturer's instructions, pour (at least) the minimum amount of peanut oil into the deep fryer, then preheat to 350-360 degrees.
Prepare batter by mixing the beer and Bisquick together until bubbly and thick.
Quickly remove rings and dunk them into batter, then place them gently in fryer basket. Lower the fry basket and fry 4-5 rings at a time for approximately 2 minutes on each side, or until golden brown.
Remove and place onion rings on a wire rack to dry. Lightly sprinkle them with salt. Let cool for approximately 3-4 minutes and serve while crisp.