Heat peanut oil in a large frying pan over medium-high heat. Add the sausages and fry for 2–3 minutes on each side until browned. Remove and set aside. Add olive oil to the pan, then add the onions (cut into large rings) and fry for 5–7 minutes until golden brown. Sprinkle the 2 tablespoons flour over the onions, add the instant gravy, and cook, stirring, for 2 minutes.
Pour the white wine into the pan and bring to a boil. Add the water and return the sausages to the pan. Add the rosemary stalk and season with salt, pepper, cardamom, nutmeg, and ginger to taste. Reduce heat to medium-low and simmer for 10–15 minutes, stirring occasionally, until the sauce becomes creamy.
While the sauce simmers, peel and dice the potatoes into roughly equal-sized pieces. Cook them in salted water for 12–15 minutes until tender. Drain and mash the potatoes as smoothly as possible with a little milk. Season the mashed potatoes with salt, pepper, and nutmeg.
Serve the sausages and onion sauce with the mashed potatoes.
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