Cut the top off the head of garlic and drizzle with oil and sprinkle with salt and pepper. Wrap in foil and place in 400 degree oven for 1 hour. Squeeze the cloves out of the husk and into a small bowl and mash with a little salt. Reserve for adding to your potatoes.
For the potatoes:
Peel potatoes and cut into several pieces. Cover with cold water in a sauce pan and heat over high until boiling. Reduce heat and simmer 20 minutes or until completely tender. Mash with the butter, roasted garlic, milk, and salt and pepper to desired consistency and taste.
For the sausages and gravy:
Cook the sausages in a skillet over medium heat and browned and cooked through. Reserve to the side covered with foil to keep warm.
Add a tbsp of oil to the dripping if needed, then add the onions. Cook over medium low until the onions start to brown and caramelize a bit. (True caramelization takes a long time, so just get them started). You want to cook them about 15 minutes.
Add the beer and let most of it boil off, then add the stock and soy sauce. Add sugar if using. Let it simmer for 5-10 minutes and taste and adjust the seasoning. Add the cornstarch mixture and whisk, cooking until it thickens the gravy.
Serve the sausages over the potatoes and top with gravy.