Heat the oil in a pot on medium heat until it reaches 350 degrees.
Cut the top and bottom off the onion then peel the skin off. Cut the onion into 1/2 inch thick rounds and gently separate each into rings.
Toss the onions in flour and then set aside.
Whisk together the flour, paprika, garlic powder, salt, and baking powder. Slowly stir in the water until a thick smooth batter forms. Dip the onion rings into the batter (working in batches if needed) and right into the hot oil. Fry on both sides until crispy.
Remove from the oil with a slotted spoon onto a paper towel lined plate and immediately season with salt and pepper. Serve with ketchup if desired.