Start the Fries

• Preheat the oven to 425°F.
• Soak the cut potatoes in cold water for at least 30 minutes.
• Drain and dry very thoroughly — any remaining moisture will steam instead of roast.
• Toss with olive oil and salt, spread onto a parchment-lined baking sheet in a single layer and bake, flipping halfway, until golden and crispy at the edges.

Make the Caramelized Onions

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

Cook the Burger Patties

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

Build the Wellingtons

• Preheat the oven to 350°F.
• Lightly grease the insides of two small round oven-safe bowls.
• Cut the puff pastry into roughly 7x7 inch squares and lay one square over each bowl.
• Layer in this exact order: burger patty, cheddar square, spoonful of burger sauce, pickle slices, caramelized onions, second burger patty, second cheddar square.
• Fold the pastry corners up and over the stack and pinch tightly at every seam — any weak point will open during baking.

Flip and Finish

• Invert each bowl to release the wrapped Wellington onto a parchment-lined baking sheet, seam-side down.
• Brush generously all over with egg wash.
• Sprinkle with sesame seeds and flaky salt.

Bake

• Bake at 350°F until deep golden brown and the pastry is crisp all over.
• If the top is browning too fast, tent loosely with foil.
• Rest before slicing.

Serve

• Slice in half and serve immediately with crispy fries and burger sauce on the side.