Prepare the Sausages and Gravy:

• Cook the Sausages:
  Heat the olive oil in a large skillet over medium heat. Add the sausages and cook, turning occasionally, for 10-12 minutes until browned and cooked through. Remove the sausages and set aside.
• Caramelize the Onions:
  In the same skillet, melt the butter and add the sliced onions. Cook for 10-15 minutes, stirring occasionally, until the onions are soft and caramelized.
• Make the Gravy:
  Sprinkle the flour over the caramelized onions and stir for 1-2 minutes to cook out the raw flour taste. Gradually add the beef stock and red wine (if using), whisking to prevent lumps. Add Worcestershire sauce and simmer for 5-7 minutes until the gravy thickens. Season with salt and pepper. Return the sausages to the skillet and simmer for a few minutes to heat through.

Prepare the Mustard Mash:

• Cook the Potatoes:
  Place the diced potatoes in a large pot of salted water. Bring to a boil and cook for 15-20 minutes, or until the potatoes are fork-tender. Drain well.
• Mash the Potatoes:
  While the potatoes are still warm, mash them using a potato masher or ricer. Add the butter, warmed cream, and milk, stirring until smooth and creamy.
• Add the Mustard:
  Stir in the Dijon and whole-grain mustard. Season with salt and freshly ground black pepper to taste. Adjust the consistency with a little extra milk if needed.

Assemble and Serve:

• Plate the Dish:
  Spoon the mustard mash onto a plate or serving dish, top with the sausages, and generously drizzle the onion gravy over the top.
• Garnish and Enjoy:
  Garnish with fresh parsley or thyme if desired and serve hot.