When ready to cook, start the Traeger grill on Smoke with lid open until fire is established (4 to 5 minutes). Set the temperature to High and preheat, lid closed, for 10 to 15 minutes.
Prick potatoes all over with a fork and place on a foil-lined rimmed baking sheet. Place garlic on a piece of foil and drizzle with oil; season with salt and pepper. Fold edges of foil up and over garlic and crimp to close creating a tight packet.
Place on baking sheet with potatoes and put on grill. Roast potatoes until tender (about 65 to 75 minutes).
Remove potatoes from grill and place in a large bowl. Cover with foil to keep warm.
Keeping grill set to High, preheat a large cast iron skillet on grill with lid closed for 10 minutes.
Heat 1/2 Tbsp oil in skillet. Add sausages and close lid.
Cook for 4 to 5 minutes each side and cook until sausage reaches an internal temperature of 150 degrees F (about 4 to 5 more minutes).
Remove sausages onto a plate and cover with foil to keep warm.
You will need 2 tbsp oil left in the skillet. Add onion and garlic and cook until golden brown (about 4 minutes). Add flour and mix through.
Add 3/4 cup of beef broth and 3/4 cup stout to mixture. Mix in the onion and add remaining beef broth (1 1/4 cup). Mix until combined.
Close lid and cook stirring every few minutes until the gravy thickens but is slightly thinner than you want - it will thicken as it cools.
Season with salt and pepper. Cover with foil to keep warm and set aside.
Halve potatoes lengthwise, then use a fork to scrape flesh into a saucepan. Squeeze garlic cloves from skins into pan. Smash mixture with a potato masher until mostly smooth with only a few lumps.
Combine half-and-half and butter in a large measuring cup and pour 1/2 cup into potato mixture, stirring with a wooden spoon until incorporated and smooth. Stir in the remaining 1/2 cup of half-and-half mixture. Season generously with salt and pepper.
Serve sausages with plenty of gravy and mashed potatoes. Top mashed potatoes with chives if desired. Enjoy!