Thinly slice the onion into rounds, then separate into rings. Thinly slice the spring onion. Thinly slice the cabbage with a sharp knife or mandoline. Put the cabbage and 1 tsp red wine vinegar in a bowl, season with salt and pepper and toss to coat. Drain the haloumi and dry on paper towel, then cut in half horizontally.
Step 2 — Crumb haloumi
Whisk 1 egg in a shallow bowl. Put 1 tbs plain flour in a separate shallow bowl. Combine the breadcrumbs and sesame seeds in a third bowl. Dust the haloumi with the flour, shake off the excess, then dip in the egg and coat in the breadcrumb mixture. Add any remaining flour to the remaining breadcrumb mixture.
Step 3 — Fry onion rings
Preheat the grill to high. Add an extra 2 tbs plain flour (see Make it faster) to the breadcrumb mixture, season with salt and pepper and stir to combine. Dust the onion rings with the flour mixture. Heat 1cm vegetable oil in a medium frypan over medium-high heat. Cook the onion rings, turning occasionally, for 6-7 mins until crispy. Remove from the pan and drain on paper towel.
Step 4 — Cook haloumi schnitzels
Meanwhile, heat 1cm vegetable oil in a separate medium frypan over medium heat. Cook the haloumi schnitzels for 2-3 mins each side until golden and cooked through. Remove from the pan and drain on paper towel. Season with salt and pepper. Cut the buns in half. Put on an oven tray, cut-side up, and grill for 1-2 mins until lightly toasted.
Step 5 — Serve up
Cut the haloumi schnitzels in half on an angle. Spread the bun bases with 1 tbs mayonnaise. Layer with some of the cabbage slaw and the haloumi schnitzels. Drizzle with 1 tbs mayonnaise, then scatter over the spring onion and sandwich with the bun tops. Serve with the onion rings and remaining slaw. Enjoy!
Step 6 — Make it faster
Short on time? Instead of tossing the onion in the flour mixture, simply cook the sliced onion in oil for 3-4 mins until softened. Add the onion to the burgers instead of serving on the side.