Thin slices of rare beef and a river of golden gravy is a kitchen essential. This less traditional method of roasting is my default. The lower temperature makes for supremely tender meat and elevates lesser cuts of meat, making it great for bottom round as well as the sirloin suggested here, which is for more of a special treat. — Anne Bell, Low Carb Revolution : Comfort Eating for Good Health.
For an even richer flavor, you could use a little Madeira or medium sherry in place of the same amount of red wine here.
Preheat the oven to 300 degrees F
Heat the oil in a large frying pan with a heatproof handle over high heat. Use a tea strainer or small sieve to dust the fat and underside of the roast with the mustard powder, and season it. Sear on all sides until golden — the ends, too — it should be well browned because you’re roasting at a low temperature. Brush half the grainy mustard over the bottom of the meat with a pastry brush, and the rest over the top. Transfer the beef in the frying pan to the oven and roast for 50 minutes for rare, then remove from the oven, loosely cover with foil, and leave it to rest for 20 minutes. (I have a habit of forgetting that the handle of the pan is hot, so leaving a towel wrapped around it isn’t a bad idea.) Thinly carve and serve with the gravy the cold meat is also great sandwich material.
Heat the oil in a large saucepan over medium heat, add the vegetables, and cook for 30 to 35 minutes, stirring occasionally, until really well browned, almost black. Add the garlic a few minutes before the end, and then gradually pour in the red wine, and cook for a couple of minutes. Stir in the stock in two or three additions, add some strong seasoning, and simmer for about 5 minutes. Pass the gravy through a sieve, pressing out as much as possible from the vegetables. If it tastes at all thin, you can simmer it for a few minutes longer, and check the seasoning. If you let it stand for any length of time, there may be a little oil on the surface, which you can skim off. Add any juices given out when you slice the beef.
Thickened gravy: If you want to thicken the gravy slightly, add 2 teaspoons flour to the pan after cooking the vegetables, then add the wine and cook, stirring, for a couple of minutes until it thickens.