In a large pot fitted with a lid, cover the potatoes in cool water and bring them to a boil. Starting in cold water allows for even cooking.
Meanwhile, in a 12-inch skillet, over medium-high heat, saute sausages for about 4 minutes. Turn them over and cook for an additional two minutes or until done. Remove from heat and set aside.
In a small pan or microwave safe bowl, gently warm the butter and milk but do not scald. This will be for the mashed potatoes. The warmed ingredients make light and fluffy mashed potatoes over gluey or pasty mashed potatoes.
Next, add two tablespoons of butter to the skillet and saute the onions until tender.
Next, form the roux by sprinkling flour over the onions and allowing it to coat the onions. Once it's coated and begins to dissolve, add the vegetable stock and the beer to the pan and bring to the boil. This will boil off most of the alcohol leaving behind the rich, deep, flavor of the ale.
Add sausages back into the skillet while the sauce thickens.
Adjust your seasonings by adding salt and pepper according to your tastes and simmer on low, covered.
When the potatoes have become tender enough to easily be pierced by a fork, drain the water and mash to desired texture. Add in warmed butter and milk, salt, and pepper and stir to combine. Taste and re-season as necessary.
As a garnish, you may choose to sprinkle parsley over the sausages and gravy for plating. This adds a beautiful brightness to the dish.