Heat a large saucepan over a medium-high heat and add the butter and 1 tbsp oil. Once melted, add the onions and brown sugar and cook for 10 minutes, stirring frequently, until softened.
Add the balsamic vinegar along with a splash of the cider and let the liquid cook into the onions. Gradually add more of the cider, continuing to cook, until all the liquid has been added and cooked down. Continue to cook for another 8-10 minutes until the onions are caramelised.
Add the mushrooms and garlic to the caramelised onions and stir through. Season well and cook until the mushrooms are soft. Add the beef stock and bring to a boil. Add the pasta, bay leaves and half the thyme and cook, stirring often, for 15-20 minutes until most of the liquid has been soaked up by the pasta and the pasta is al dente.
Meanwhile, add the remaining oil to a frying pan and place over a medium heat. Once hot, add the sourdough pieces and cook for 3-4 minutes until lightly golden. Add the leaves from the remaining thyme and continue to cook until the breadcrumbs are deeply golden in colour. Transfer the breadcrumbs to a kitchen towel.
Once the pasta is done, stir in the yogurt followed by most the gruyère cheese until melted. Spoon into dishes and serve topped with the remaining cheese, breadcrumbs and thyme sprigs.