For the spiced ketchup: In a bowl, whisk together the ketchup, mustard and Sriracha, stirring well. Then refrigerate until serving.
For the onion rings: Preheat a deep-fryer to 375 degrees F with vegetable or peanut oil, waiting until heated. Next, drain the ice water from the cut onions, ensuring the water has drained. Next, in a bowl, blend the flour, salt and pepper, mixing well. Then add one-third of the onions. Mix the onions well, tap off excess flour and then add to the fryer, cooking until crisp, about 2 minutes. After removing, allow to drain excess oil on paper towels. Repeat with the remaining onions. Serve with the spiced ketchup.