Start by heating olive oil in a Cast Iron Dutch Oven over medium high heat.
Once the oil has heated up add in the sausage and cook until both sides are browned. Once the sausage has been browned remove them from the pot and set aside.
Next, add the butter to the dutch oven allowing it to fully melt while deglazing the bottom of the pot.
Once the butter has melted, add the thinly sliced onion and sauté until they've softened and are just beginning to turn translucent.
Add the garlic, stir to mix and sauté for about another 2 minutes.
Now pour in the flour, mustard powder, parsley, thyme, salt, and pepper. Stir this mixture into the onions and continue to sauté for another 2-3 minutes or until the seasoning becomes aromatic.
Lastly, stir in the Worcestershire Sauce, Beef Stock, and Guinness Stout.
Add the sausage back in and simmer for about 30 minutes.
Serve with a side of Green Peas, Mashed potatoes, and fried cabbage.