Preheat the oven to 220C, fan-forced. Line an oven tray with baking paper. Cut the unpeeled sweet potato into 1cm-thick chips. Thinly slice the tomato and onion. Combine half the barbecue seasoning and 3 tsp olive oil in a large bowl and season with salt and pepper. Add the steaks and turn to coat.
Step 2 — Roast sweet potato chips
Put the sweet potato on the lined tray. Drizzle with 3 tsp olive oil, scatter over the remaining barbecue seasoning, season with salt and pepper and toss to coat. Roast, turning halfway, for 20-25 mins until golden and tender.
Step 3 — Caramelise onion
Meanwhile, heat 2 tsp olive oil in a medium frypan over medium heat. Cook the onion, stirring regularly, for 5 mins or until soft. Stir in 2 tsp red wine vinegar and 2 tsp sugar. Reduce the heat to low and cook, stirring occasionally, for 3-4 mins until caramelised. Remove from the pan and cover to keep warm. Wipe the pan clean.
Step 4 — Pan-fry steaks
Return the pan to high heat until hot (see Kitchen notes). Reduce the heat to medium-high and cook the steaks, turning regularly, for 4-6 mins for medium-rare, or until cooked to your liking. Remove from the pan and rest for 3 mins. Meanwhile, cut the buns in half. Put on an oven tray, cut-side up, and bake for 5 mins or until toasted and golden.
Step 5 — Serve up
Slice the steaks. Spread the bun bases and tops with the mayonnaise. Layer the bases with the tomato, steak and caramelised onion. Sandwich with the bun tops and serve with the sweet potato chips. Enjoy!
Step 6 — Kitchen notes
It's important to have the pan very hot before adding the beef, otherwise it may stew slightly and become tough. To ensure tender beef, slice the steaks against the grain.