Cook bacon in a casserole pot over medium heat, turning occasionally, until evenly browned, about 10 minutes. Add butter, onions, and pinch of salt, and cook until onions are soft and translucent. Add garlic, and spring onions, cook stirring for 2 minutes.
Pour in stout, raise heat to high, and cook, stirring occasionally, until stout has reduced by about 75%. While stout is reducing, season with thyme, parsley, black pepper, salt, and pinch cayenne. Once reduced, add potatoes and stock and bring to a simmer.
Preheat the oven to 150º C).
Prick sausage on both sides, and add to the pot on top of the other ingredients.
Cover and place pot into the preheated oven. Cook for about 2 1/2 hours. Remove the lid, and continue cooking uncovered for about 1 more hour.
Remove from oven and skim excess fat from top before serving. Garnish top with green onions, if desired.
