Method

Using a food processor, pulse the flour, sugar and pepper to combine, then add the butter and pulse until no large pieces of butter are visible. Add the grated Cheddar and blend until the dough comes together in a ball.
Immediately turn the dough out onto a lightly floured work surface and roll out into a circle just under 1.5cm thick. Line a lightly floured 23 cm loose-bottom fluted tart pan and press in, trimming the edges. Chill this until firm, about 2 hours.
Preheat the oven to 180 °C. Dock the shortbread with a fork and bake until golden brown, about 30 minutes. Cool the shell before filling. Leave the oven at 180 °C.
For the filling, melt the butter in a sauté pan over medium heat and add the onions and thyme. Sauté the onions until they are caramelised, about 20 minutes. Add the balsamic vinegar and stir until the liquid has evaporated. Season lightly and remove the pan from the heat.
In a mixing bowl, beat the cream cheese by hand to soften. Add the egg yolk and beat in the whole egg. Stir in the Cheddar cheese and parsley and then stir in the caramelised onions. Scrape this into the cooled crust and bake for 35-40 minutes until the outside edge of the cheesecake just begins to soufflé a touch. Allow the cheesecake to sit for 10 minutes before slicing.
The cheesecake can be enjoyed warm or at room temperature.