Place the bacon pieces into a medium pot and fry until crispy.
Remove the bacon from the pan, leaving the bacon fat in the pan. If the bacon left a large amount of fat, you can remove some of it. Try to leave around 4 tablespoons in the pan to use for the sautéing.
Add the diced onion to the bacon fat (or butter, if you're not using bacon) and sauté until softened. Add in the sage, garlic, and butternut cubes. Sauté for 1-2 more minutes to brown the cubes slightly and bring out more flavour.
Pour in the stock, cover with a lid, and bring to a simmer. Simmer for 10-15 minutes, or until the butternut cubes are soft enough to smash. At this time, you'll want to start cooking your pasta, if you haven't already.
Remove from the heat, and add in the milk. Use an immersion blender to puree it all together. You can use a food processor or regular blender here too, but an immersion blender is far easier.
Taste the sauce and add salt/pepper if needed. Add the cooked pasta into your pot and mix to coat.
Serve with the bacon bits on top.