Preheat the oven to 220C, fan-forced. Line 2 oven trays with baking paper. Cut the unpeeled sweet potatoes into 2cm-thick wedges. Put the wedges on a lined tray, spray with olive oil and season with salt and pepper. Roast on the upper oven shelf for 20-25 mins until golden and tender.
Step 2 — Roast onion and capsicum
While the wedges are roasting, slice the onion and capsicum. Put the onion and capsicum on the remaining lined tray, drizzle with 2 tsp olive oil, 2 tsp white wine vinegar and 1 tsp sugar and toss to coat. Roast on the lower oven shelf, tossing occasionally, for 15-18 mins until tender and caramelised.
Step 3 — Prep slaw
Meanwhile, thinly slice the cabbage with a sharp knife or mandoline. Shred the carrot using a julienne peeler or coarsely grate with a box grater. To make the dressing, put 2 tsp extra virgin olive oil, 1 tsp white wine vinegar and ½ tsp dijon mustard in a large bowl, season with salt and pepper and whisk to combine.
Step 4 — Make and cook patties
BEEF MINCE OPTION Put the beef mince and barbecue seasoning in a large bowl. Using damp hands, combine well, then shape into 2 equal patties, slightly larger than the base of a bun. Heat 2 tsp olive oil in a medium frypan over medium-high heat. Cook the beef patties for 3 mins each side or until cooked through.
Step 5 — Toast bread rolls
While the patties are cooking, halve the bread rolls through the middle. Put on an oven tray, cut-side up, and bake for 4-5 mins until lightly toasted. (If you don't have space in the oven, toast the rolls after the onion and capsicum are cooked.)
Step 6 — Get ready to serve
Add the cabbage and carrot to the dressing and toss to coat. Spread the bread rolls with the aioli. Divide the patties, onion, capsicum and some of the slaw among the bases, then sandwich with the tops. Serve with the wedges and remaining slaw.