Preheat the oven to 220C, fan-forced. Line an oven tray with baking paper. Cut the unpeeled sweet potato into 1.5cm-thick fries. Thinly slice the onion. Cut the bread rolls in half. Coarsely grate the cheese. Put the beef mince and half the Mexican seasoning* (see Kitchen notes) in a large bowl. Using damp hands, combine well, then shape into 2 equal portions.
Step 2 — Bake sweet potato fries
Put the sweet potato on the lined tray, drizzle with 2 tsp olive oil, season with salt and pepper and toss to coat. Bake for 23-25 mins until golden and tender. Meanwhile, heat 2 tsp olive oil in a small frypan over medium-high heat. Cook the onion and a pinch of salt, stirring, for 4-5 mins until soft and slightly caramelised. Remove from the pan and reserve the pan.
Step 3 — Cook cheesy beef patties
Heat 2 tsp olive oil in a medium frypan over medium-high heat (see Kitchen notes). Add the beef and press down on each with a spatula to flatten into a patty slightly larger than the base of a bread roll. Cook for 2-3 mins on one side then, flip over, scatter the patties with the cheese and cook for 2-3 mins until the patties are cooked through and cheese is melted. Remove from pan.
Step 4 — Bake rolls and fry eggs
While the patties are cooking, put the rolls on an oven tray, cut-side up, and bake for 5-6 mins until lightly toasted. Heat 1 tsp olive oil in the reserved pan over medium heat. Crack 2 eggs into the pan and cook for 3-4 mins until just set or cooked to your liking. Remove the pan from the heat.
Step 5 — Serve up
Combine 1 tsp red wine vinegar and 1 tsp olive oil in a large bowl and season with salt and pepper. Add the spinach and toss to coat. Spread the rolls with the mayonnaise. Layer the bases with the beef patties, onion, eggs and some of the spinach, then sandwich with the tops. Serve with the sweet potato fries and remaining spinach. Enjoy!
Step 6 — Kitchen notes
Our Mexican seasoning has a mild chilli kick, so reduce the amount if you prefer. ~ 'Smash' burgers are formed by flattening the beef portions in the pan rather than shaping them into patties before cooking. Make sure your pan is hot before adding the beef so it browns nicely. Top the beef with baking paper before flattening to prevent the spatula sticking, if you like.