Place the beef and pork belly in the bowl of a food processor. Pulse briefly until just minced. Transfer to a bowl. Add the breadcrumbs and Tabasco. Season. Use your hands to mix until combined and slightly sticky. Cover and place in the fridge for 30 minutes to chill. Divide into 4 portions and shape into patties.
Meanwhile, for the crispy onion rings, place the onion in a bowl. Pour over the milk. Set aside for 30 minutes to soak. Place flour in a bowl. Season well. Add enough oil to a large saucepan to reach 5cm up the side. Heat vegetable oil over medium-high heat to 180C on a cook's thermometer. Drain the onion rings and toss in flour. Deep-fry, in batches, for 2-3 minutes or until crisp and golden. Drain on paper towel.
Preheat a barbecue flatplate on medium-high. Brush the patties lightly with olive oil. Cook patties for 4 minutes each side or until lightly charred and cooked through. Transfer to a plate.
Divide spinach, tomato and patties among roll bases. Top with sauce, onion rings and roll tops.