Heat the butter and oil in a large pot over medium heat on the stovetop. Add the diced celery, carrot, onion and pancetta and sauté until the vegetables have softened.
Turn the heat up to high and add the pork mince and beef mince. Cook until the mince has browned and all of the liquid has evaporated. Add rosemary and thyme sprigs.
Add the bay leaves and season with salt and pepper. Stir to combine. Turn the heat down to low. Cover with a lid and allow to cook slowly for a minimum of 1 hour (up to 2-3 hours).
Remove half of the bolognese sauce from the pot and place into a large frying pan.
The remainder of bolognese sauce can be stored in the fridge or frozen for later use.