Step 1
Preheat the oven to 200C fan-forced (220C conventional) and line an oven tray with baking paper.
Step 2
Melt the butter in a lidded frying pan over medium heat. Add the onions, season with salt and pepper, increase heat to high, cover with the lid and cook for 6 minutes, stirring occasionally.
Step 3
Add the sugar, 1 tablespoon of the worcestershire sauce and the stock and continue to cook for 10 minutes, or until the onions are soft and caramelised.
Step 4
Place the frozen fries on the prepared tray in a single layer and place in the oven for about 20 minutes, or until golden, turning halfway through.
Step 5
Meanwhile, place the mince in a large bowl, add the mustard, thyme and the remaining worcestershire sauce. Mix well to combine. Divide the mince into four patties.
Step 6
Heat a large nonstick frying pan over high heat. Add the patties and cook for 8 minutes, turning halfway through the cooking time. Place a cheese slice on top of each patty. Cover the pan with a lid, remove it from the heat, and set it aside to allow the cheese to melt.
Step 7
Spread aioli on the base of each bun and add a handful of watercress. Top each with a patty, melted cheese facing up, and add a spoonful of caramelised onions. Place the bun lid on top and serve the burgers with hot fries on the side.
