In a medium mixing bowl, whisk flour with salt and pepper and then pour in beer and Worcestershire sauce. Whisk it all together until smooth.
Taste the batter to make sure you have enough salt.
Cover and set aside for about 30 minutes (if you have the patience).
Slice the onions into thick rings that are between 1/4-inch and 1/2-inch thick. Pat them dry with a paper towel to soak up excess onion juice.
Preheat the oil in a large skillet, deep pan, or in a fryer, on medium-high heat. (Do not use a shallow pan so that you minimize the splatter.) Make sure to preheat to and maintain the oil at 350° and 365°.
Line a large wire rack with paper towels to soak up the excess oil after frying and place it on the counter nearby.
Dip the onion rings into the batter and carefully add them to the hot oil. Fry until they are golden brown and flip. Give them a couple more minutes to fry until golden brown on the other side.
Fry them in batches and don't overcrowd the pan/pot.
Use metal tongs of a metal slotted spoon to take out the fried onion rings and place them onto the prepared wire rack to soak up excess oil.
Give it a couple of minutes to cool down and serve.