Preheat the oven to 200 degrees Celsius.
Start by cooking your sausages. Dry fry them in a large nonstick pan for 10 minutes until golden brown. Remove from the pan.
Peel and finely chop your onion. Quarter your mushrooms. Put half of the olive oil in the pan and put on a medium heat. Add the onions and mushrooms.
Cook for 5 minutes until the mushrooms are starting to turn golden. Slice the sausages and add them to the pan too.
Sprinkle in the flour and stir well. Add the stock a little at a time, stirring each time.
Add the barbecue sauce, rosemary, thyme and bay leaf. Stir, then turn the heat to low. Simmer for 10 minutes.
Turn off the heat and add the peas. Stir, then tip into a large ovenproof dish.
Top with the hash browns, then sprinkle over the Parmesan. Bake in the oven for around 50 minutes, until the hash browns are crisp on top.