Warm oil over medium heat in a large nonstick skillet. Add sausages and cook, until browned all over and cooked through (rolling frequently to prevent burning), about 10-12 minutes. Transfer to a paper towel-lined plate and pour off all but 1 tablespoon fat from the pan.
Add butter; once bubbly, add in the onion and stir to coat. Continue to cook for about 10 minutes until the onions are translucent, golden, and begin to caramelize.
Add in the garlic and cook until fragrant, about 30 seconds.
Add flour and mix through until completely combined.
Add about 3/4 cup of beef broth and mix into the onion so it becomes a sludge. Then add remaining beef broth and mix until combined.
Simmer, stirring, until the gravy thickens but is slightly thinner than you want – it will thicken more as it sits.
Season with salt and pepper to taste.
Transfer the cooked sausages back to the pan just to warm through.
To serve, mound some mashed potatoes on a plate, top with 2 sausages, drizzle with gravy, and some sweet baby peas on the side. Yum!
