Preheat an oven to 140°c/120°c fan (284°f/248°f)
Place the veg oil into a large saucepan then set it over a medium-high heat.
Let the oil heat up for a minute or so then add in the pancetta. Fry until crispy, stirring regularly then transfer to a bowl with a slotted spoon (leave any fat in the pan!).
Next, add half of the beef mince into the pan then season with a pinch of salt. Fry until well browned, breaking the mince up with a spatula/spoon as you do so.
Using a slotted spoon, remove the browned mince from the pan & transfer to the pancetta bowl, leaving any fat in the pan. Repeat with the remaining mince.
Turn the heat down to medium then add in the diced onion, carrot & celery. Season with a pinch of salt then cook for 4-5 minutes, until soft, stirring frequently. Stir in the grated garlic & chopped thyme then cook for another minute.
Add in the tomato paste then cook for 4-5 minutes, stirring almost constantly then stir the mince & pancetta back in.Cook the tomato paste until it darkens in colour. This caramelises it, adding extra flavour to our sauce. Pour in the red wine, deglaze the pan then cook for 3-4 minutes, until almost completely reduced.To deglaze the pan, scrape the bottom with a spatula/wooden spoon to release any browned bits that are stuck on. Next, add in the tin of tomatoes & the stock cube. Break the tomatoes up with a spoon then fill the empty tin up with cold water & pour into the sauce.
Stir in the milk & add in the bay leaves, bring the sauce up to a gentle simmer then season with salt & pepper. Transfer the pan to the oven & cook uncovered, for 2½ - 3 hours until the meat is tender & the sauce has thickened.Don't let the sauce boil as the sauce won't emulsify properly. Make sure to give the sauce a stir every 30-45 minutes whilst it's in the oven. Once the sauce is cooked, place it back on the stove over a low heat then stir in the Worcestershire sauce. Give the sauce a taste, to check the seasoning then add more salt & pepper if needed.
Take the bolognese off the heat & leave to cool. Transfer to an airtight container then refrigerate overnight before using.