Bring a large pot of water to boil. To make the Ballotine filling, cut the mushrooms in half and add to a food processor with the chorizo sausage. Blend until it comes together as a paste.
Get your chicken thigh fillets and ensure they are uniform in thickness. If the thighs are particularly uneven, use a meat cleaver or rolling pin to flatten the thicker parts slightly, but don’t over do it.
Place a large sheet of foil on your bench, and put a sheet of baking paper on top of the foil. Place your jamon or pancetta slices down on the baking paper, each slice slightly overlapping the other. Place chicken thigh fillets (skin side down), on top of the jamon or pancetta in a row.
Next, spread the mushroom and chorizo mix across the length of your chicken, slightly in from the edge.
Using both the foil and baking paper, start to roll your chicken over itself, until it forms a tight cylinder with the foil and baking paper completely wrapping the outside of the Ballotine. Pinch both ends of the foil and twist them tight like the end of a lolly wrapper so that the Ballotine is completely sealed.
Place the rolled chicken parcel into the medium-large sized pot filled with boiling water and poach for around 20 minutes, or until it is just cooked through.
After 20 minutes, carefully remove the parcel from the boiling water and unwrap the foil and paper parcel.
Add butter and oil to a frying pan, and once hot add the cooked Ballotine to crisp up the outside jamon or pancetta layer. At the same time, quarter your Swiss Brown mushrooms and add them to the same pan to cook alongside the Ballotine in the butter and olive oil.
Baste the Ballotine with the mushroom butter juices for 2-3 minutes, and once golden and crispy, remove from the pan and leave to rest while you make the salad.
To make the salad: Place the beans, chilli, onion, chives and chopped olives in a bowl. Add the cooked mushrooms, but save the mushroom juice. Add a generous squeeze of kewpie mayo and mix until everything is well combined.
To assemble the dish, spoon the bean salad onto a plate. Cut the chicken Ballotine into thick 3-4cm slices, and add to the plate. Drizzle over the mushroom butter jus and garnish with extra chopped chives.