Preheat the oven to 180°C (350°F). Lightly grease a small baking dish that will hold the brisket slices in a snug layer.
Heat the olive oil and butter in a pan over medium heat. Add the sliced onion and cook for 6–8 minutes, stirring often, until softened and lightly golden.
Stir in the minced garlic, smoked paprika, dried thyme, salt, and black pepper. Cook for 30–60 seconds until fragrant.
Add the tomato paste and cook for 1 minute, stirring to coat the onions. Pour in the beef stock and red wine (if using), then stir in the Worcestershire sauce and Dijon mustard.
Bring the mixture to a gentle simmer and cook for 3–4 minutes. If you prefer a slightly thicker sauce, stir in the cornstarch mixed with cold water and simmer for another 1–2 minutes until lightly thickened. Taste and adjust seasoning.
Arrange the sliced brisket in the prepared baking dish, slightly overlapping the slices. Pour the hot onion pan jus evenly over the brisket, making sure all slices are moistened.
Cover the baking dish tightly with foil and bake for 20–25 minutes, until the brisket is warmed through and tender. For thicker slices, add an extra 5–10 minutes.
Remove the foil and spoon some of the sauce over the top of the brisket. If you’d like lightly caramelized edges, bake uncovered for an additional 5 minutes.
Garnish with chopped fresh parsley and serve the brisket with plenty of the onion pan jus over potatoes and green vegetables, if using.