Prepare the potatoes: peel if desired and cut potatoes into even chunks. Place in a pot, cover with cold water, bring to a boil, and cook until fork-tender. Drain well and return to the pot.
Make the mashed potatoes: to the drained potatoes add 1 tablespoon butter, 2 tablespoons sour cream, ½ teaspoon salt, and ¼ teaspoon black pepper. Mash until smooth and keep warm.
Prep for the gravy: slice the medium yellow onion. Measure 1.25 cups beef stock and 1 tablespoon cornstarch, then whisk the cornstarch into the beef stock until smooth; set aside.
Brown the sausages: heat 1 tablespoon olive oil in a large skillet over medium-high heat. Add the 4 pork sausages and brown them on all sides, turning occasionally, until cooked through. Transfer the sausages to a plate and set aside.
Cook the aromatics: reduce the heat to medium and add 1 tablespoon butter to the same skillet. Add the sliced onion and 1 teaspoon minced garlic and sauté, stirring occasionally, until the onion is softened and nicely browned.
Make the gravy: pour the beef stock/cornstarch mixture into the skillet, scraping up any browned bits. Simmer, stirring occasionally, until the sauce thickens to your desired consistency.
Season and finish: stir in ½ teaspoon salt and ¼ teaspoon black pepper. Return the sausages to the skillet and simmer for a minute or two so they heat through and are coated with gravy.
Serve: spoon the mashed potatoes onto plates, top with the sausages, and spoon the gravy over the sausages and potatoes. Serve immediately.