Heat the vegetable oil in a large heavy based pot - cast iron is best - and cook the onions together over a medium-low heat. Keep stirring to make sure they don't catch and burn and cook until they've softened and reduced in volume. The more moisture you get out the less likely your giant onion ring will be soggy!
Mix together the beef mince, mustard and seasonings.
In a frying pan heat a splash more oil then fry the beef over a high heat until browned and cooked through.
Take a large springform tin and place a small circular bowl in the middle. Lay some clingfilm down and lay half of the cooked onion mix in a ring around it.
Top with grated mozzarella and the beef mince and finish with the remaining onion to seal in the burger - cheese layer.
Freeze until solid - roughly three hours.
Meanwhile, prepare trays of the flour, eggs and breadcrumbs.
Preheat the oil to 180°C in a large and very sturdy pot.
Season the flour and press to the sides of the onion ring. Shake off excess and dip in the egg mixture, making sure it's completely covered, before dipping in the breadcrumbs, patting them on to ensure an even coating.
Very, very carefully lower the ring into the oil - a trivet placed at the bottom can stop the ring getting scorched.
If the oil doesn't reach the top then baste it with a spoon. Cook until golden and crispy, then (being very careful) remove from the oil and place on a tray in the oven for around 30 - 40 minutes to cook through.
While it's baking, whisk together all the ingredients for the sauce.
Serve the ring in slices with lashings of the sauce and the usual burger accoutrements.