Soak and dry the fries

• Preheat the oven to 220 °C (425 °F).
• Cut the potatoes into fries and soak them in cold water for at least 30 minutes.
• Drain and dry them very thoroughly with a kitchen towel — any moisture will steam instead of roast.

Bake the fries

• Toss the dried fries with olive oil and salt.
• Spread them in a single layer on a parchment-lined baking sheet.
• Bake 25–35 minutes, flipping halfway, until golden and crispy at the edges.

Caramelize the onions

• Heat the olive oil in a skillet over medium heat, add the onions and salt and cook 5–7 minutes until softened.
• Reduce the heat to medium-low and cook a further 10–20 minutes, stirring occasionally and adding a splash of water if they stick.
• Add the sugar if using and cook until jammy and deeply golden, then set aside to cool slightly.

Sear the patties

• Season the patties generously with salt and pepper.
• Sear in a hot pan 2–3 minutes per side until just cooked through — they will finish in the oven.
• Rest briefly and let cool slightly before building.

Build the Wellingtons

• Preheat the oven to 175 °C (350 °F) and lightly grease the insides of two small round oven-safe bowls.
• Cut the puff pastry into roughly 18 cm squares and lay one square over each bowl.
• Layer in this order: burger patty, cheddar square, a spoonful of burger sauce, pickle slices, caramelized onions, second burger patty, second cheddar square.

Wrap and seal

• Fold the pastry corners up and over the stack.
• Pinch tightly at every seam — any weak point will open during baking.
• Invert each bowl to release the wrapped Wellington onto a parchment-lined baking sheet, seam-side down.

Brush and top

• Brush the pastry generously all over with egg wash.
• Sprinkle with sesame seeds and flaky salt.

Bake

• Bake at 175 °C (350 °F) for 30–35 minutes until deep golden brown and the pastry is crisp all over.
• If the top browns too fast, tent loosely with foil.
• Rest 5 minutes before slicing.

Serve

• Slice each Wellington in half.
• Serve immediately with the crispy fries and burger sauce on the side.