Heat 2 Tbs butter in a pan and the same amount of oil in a pan. Add sausages cook them for around 4 minutes, remove.
Give remaining 4 Tbsp butter into the same pan. Add onions, garlic, and brown sugar. Caramelize for 20 minutes.
Make mashed potatoes, see linked recipe directions.
Prepare a cornstarch slurry by mixing 1 Tbsp vegetable broth with 1 tsp corn starch. Whisk into the onions, then add remaining broth, tamari, mustard, and a pinch of salt and pepper.
Return the sausages to the onion gravy and let simmer for another 5 minutes on low heat.
