Whisk all ingredients together in a small bowl. Set aside.
Cut ends off onion and peel, leaving onion whole. Cut onion into generous 1/4" slices. In a large bowl toss onions with a few drops of water to moisten. Sprinkle flour dredge over onion slices and toss to coat. Set aside.
Preheat a deep fryer to 350℉ or at least 3" of canola oil in a large pot or dutch oven to 350℉ using a thermometer hooked to the edge of the pot.
In a large bowl whisk together flour, herbs, salt, baking powder, onion powder and garlic powder. Whisk in buttermilk, melted butter and worcestershire sauce until smooth.
When oil is hot, scoop out 4-6 onion rings and dunk in batter to completely coat. Shake off excess batter and place onion rings in oil one at a time. Cook for approximately 2-3 minutes per side or until golden brown. Remove with tongs or a slotted spatula to a paper towel lined plate. Keep warm in a low oven while frying remaining onion rings.
Batter will thicken slightly if left to sit. Thin with extra buttermilk if necessary.