Preheat the oven to 400F/200C.
Slice the beef into 4 portions, then gently roll each one into a ball. Press each one into a patty that is 3.5" wide (they should end up around 3/4" thick). Don't overwork the meat otherwise the patty will come out chewy (think feather fingers). Generously season both sides with salt and black pepper.
Place a large heavy-based or cast-iron pan over high heat. Add a drizzle of oil to the pan, then once hot, add the patties and leave for 1 minute and 30 seconds. Flip them over and fry for another 1 minute and 30 seconds, then remove from the pan. This isn't to cook them right through, more just to get a nice crust on the outside. Leave the patties to completely cool to room temp (important otherwise they'll steam the pastry). You may need to work in batches.
Slice 4x circles from the pastry that are 1/2" wider than the patty (should end up 4.5" wide). Slice 4x larger circles approx 1" wider than the smaller circles. They need to be large enough to cover the whole burger and attach to the base circle.
Place a slice of cheese on each of the smaller circles (fold in the corners if you need to) then spread over 1 tbsp of chutney/caramelised onions. Pat the patties dry if they're notably oily/wet and place on top of the onion, then add another slice of cheese on top. Spread some sauce on top, then add the pickles on top.
Drape the larger circles over the top and use a fork to crimp the outside into the bottom slice of pastry (try to close as tightly as you can without tearing). Use a knife to slice off excess pastry and clean up the outside.
Carefully place them on a large tray with baking/parchment paper. Brush with egg wash, sprinkle with sesame seeds, then slice a couple of small steam holes in the top. Bake in the oven for 25 minutes, or until deep golden and visibly flaky/crispy.
The filling will be very hot, so rest for a few mins then tuck in and enjoy!