In a large nonstick skillet, heat the oil over medium heat. Cook until the sausages are browned all over and cooked through (rolling regularly to prevent burning), about 10-12 minutes. Pour all except 1 tablespoon of the grease from the pan onto a paper towel-lined dish.
Add butter to the pan. When the butter is bubbling, add the onion and toss to coat. Cook for another 10 minutes, or until the onions are transparent, golden, and starting to caramelize.
Add garlic. Cook for 30 seconds, or until the garlic is aromatic.
Mix in the flour until entirely mixed.
Mix in approximately 3/4 cup beef broth until the onion turns into sludge. Then stir in the remaining beef broth until well mixed.
Simmer, stirring occasionally, until the gravy thickens but is still slightly thinner than desired — it will thicken further as it sets.
Season with salt and pepper to taste.
Return the cooked sausages to the pan to reheat thoroughly.
Mound some mashed potatoes on a platter, top with 2 sausages, pour with gravy, and serve with some delicious baby peas on the side. Yum!