Open out the thighs and place them between two squares of baking parchment. Bash the thighs with a rolling pin to flatten them, but not so violently that the chicken flesh starts to break up.
Set out three shallow bowls. Fill one with the flour, the next with the beaten egg and the last one with the breadcrumbs. Season the thighs, then dip each one in the flour, then the egg, then the breadcrumbs. Set the coated thighs on a baking sheet.
Heat the oil and butter in a large frying pan. Cook the thighs over a medium heat for 4 mins each side. The coating should be golden and the chicken cooked through. Season, then transfer to a roasting tin lined with kitchen paper and keep warm in a low oven.
Wipe out the pan and add the butter. Cook until foaming and nutty (it will take about 3 mins for the butter to become brown), then add the capers and the rosemary and cook for 1 min more. Serve the schnitzels with the butter spooned over the top and the lemon wedges on the side.