Put the fat in a large pot for which you have a lid on a medium-high heat, then fry the bacon until the fat runs and it just starts to brown, but before the bacon turns crisp. Lift out the bacon, keeping as much fat in the pan as possible, and set aside.
Turn down the heat to medium-low, then fry the onions, stirring regularly, until soft, but without letting them take on too much colour apart from that from the bacon fat. Stir in the thyme, if using, and season with pepper.
Add the sausages, bacon, and potatoes and pour over the liquid – use more if you’d prefer a brothier dish.
Bring to a simmer, then cover, turn down the heat and cook for about 45 minutes, until the potatoes are cooked through.
Taste the gravy, season if necessary, then sprinkle with parsley, if using, and serve with cabbage and brown, Worcestershire or similar sauce.
