In a heavy-based pan or casserole dish on a medium-low heat, warm 2 tbsp of the oil, then add the sausages and turn regularly until lightly brown on all sides.
Remove the sausages from the pan - this is just a preliminary browning, and they will be cooked more later.
Back in the pan, add the remaining 2 tbsp of olive oil to the hot pork fat, then stir in the onion.
Once the onion is soft, add the sage, then, after a minute or two, the tinned tomatoes, tomato concentrate, chilli flakes and a pinch of salt.
Simmer for 10 minutes, pressing the tomatoes with the back of a spoon to break up any lumps.
Add a glass of warm water, return the sausages to the pan, and simmer, turning regularly, for 20-25 minutes (if you are using veggie sausages, adjust the timing as required), adding more water if the sauce thickens too much.
Once the sausages are cooked through, stir in the drained beans, simmer for a further five minutes, then add the greens and stir until they wilt completely into the sauce.
Taste, adjust for salt and pepper as needed, and serve.
