Preheat oven 180°C. Line an oven tray with baking paper.
Cut sweet potato into thick wedges and transfer to prepared tray. Drizzle with oil and sprinkle with paprika, thyme, and salt. Toss to coat potato in spiced oil, then arrange in a single layer. Roast wedges for 25 mins until golden and tender.
Preheat BBQ or a heavy-based skillet over high heat. Drizzle steaks with olive oil and season with a pinch of salt and pepper.
Cook steaks for 2–3 mins per side, or cooked to your liking. Split bread rolls and toast until golden.
To assemble the sandwich, layer bottom bun with spicy aioli, lettuce, and tomato. Top with steak, onion relish and top bun. Serve with sweet potato wedges.