Start by making mashed potatoes. Use your preferred method. I use my Instant Pot Mashed Potatoes recipe. Add butter and sour cream and mash well. Season with salt and pepper.
Slice the onion. Measure out beef stock and whisk it with cornstarch.
In a skillet, heat up olive oil. Brown sausages on each side. Remove from pan.
Add butter to the same skillet. Add onion and garlic. Saute until fragrant and nicely brown. Add beef stock and cornstarch mixture. Simmer until thickened into a sauce. Season with salt and pepper. Add sausages back to the gravy and heat it all up together.
Serve sausages with gravy on a bed of mashed potatoes and a side of green peas.