Place a saucepan of water on medium heat. Add in the chicken breast and once the water is boiling, reduce the heat and leave to simmer for 15-20 minutes. Remove the chicken from the pan and place on a chopping board. Shred the chicken and set aside.
Meanwhile, place a large pot on medium heat and drizzle with 1 tablespoon of vegetable oil. Once hot, add in the onions, garlic, and red bell peppers. Fry for 10 minutes until slightly softened. Add in the tomato paste and stir to combine. Season with salt and pepper.
Pour in the tinned tomatoes, malt vinegar, sweet chilli sauce, chicken stock cube, 400ml water, and 1 teaspoon chilli powder (go easy on this depending on your spice tolerance!!). Stir to combine, then add in the kidney beans and shredded chicken. Stir to combine, and leave to simmer for 10-30 minutes. Taste and season accordingly.
Now, it's time to separate the chicken chimichanga and the juices. Place a large bowl on your kitchen surface, then place a large sieve on top of the bowl. Pour the chimichanga chicken filling into the sieve and let the juice drip into the bowl below (we'll be using this later as our sauce!!). You want the chicken mixture to be as dry as possible, so use a spoon to push the liquid through (this should take around 5 minutes).
Preheat the oven to 200c fan / 425F. Slice a piece of puff pastry into a circle around 10 inches in diameter (place on a baking tray with a piece of parchment paper under the puff pastry). Using tongs, create a circular 1 inch thick layer of chimichanga chicken filling on the base of the puff pastry, leaving a 1 inch edge around the pastry (see step by step photos further up this blog or the video tutorial!).
Add a layer of mozzarella and pickled jalapenos, then top with a final layer of chicken filling, creating a dome shape. Use paper towels to soak up any excess juices!
For your second piece of puff pastry, cut out a 14 inch diameter circle. Delicately place the piece of puff pastry on top of the chicken filling, and then press and seal the top layer to the edges of the base layer. I highly recommend going around the edge of the pithivier and folding the pastry in on itself to create a sealed fold so the juices don't escape. Slice a little cross at the tip of the pithivier so the steam can escape. Brush the pastry with eggwash, then place in the oven for 30 minutes until golden brown.
Heat up your excess juices (from your bowl) in the microwave or in a pan until piping hot. Once the pithivier is puffed up and golden, remove from the oven.
Slice the pithivier and divide between plates. Drizzle over the chimichanga sauce and enjoy, my friends!