For the Roast:
Preheat oven to 450˚F. Place beef, fat side up, in a medium-sized roasting pan. Brush the beef with the mustard and sprinkle with chopped rosemary, salt and pepper. Roast for 20 minutes, and then lower the oven to 325˚F. Now cook roast to the desired doneness, allowing approximately 50 to 60 minutes more cooking time for rare; 60 to 70 minutes for medium-rare to medium (see Note)
Set roast on a platter, tent with foil and rest 15 minutes before slicing. To make the jus, set roasting pan over medium-high heat. Add wine, bring to a simmer, and scrape the bottom of the pan to lift off the brown bits. Add the stock and simmer a few minutes more. Pour jus into a sauceboat and serve it, and the quinoa (see below), with the beef.
For the Quinoa:
When the roast is 20 minutes or so from being done, make the quinoa. Place 1 1/2 Tbsp. of the oil in a medium-sized pot set over medium heat. Add the onion, bell pepper and garlic and cook 3 to 4 minutes. Add the quinoa and cook 2 to 3 minutes more. Add the stock, salt and pepper to the pot, increase the heat to high, and bring to a boil.
Cover the pot, lower heat to medium-low, and cook, undisturbed 15 minutes, or until the quinoa is tender and the stock has been absorbed by it.
While the quinoa cooks, heat the remaining 1 1/2 Tbsp. oil in a wide skillet set over medium to medium-high heat. Add the mushrooms and rosemary and cook until mushrooms are tender and the moisture has been simmered out of them, about 5 to 7 minutes. Season with salt and pepper and remove from the heat.
When the quinoa is cooked, mix in the mushrooms and parsley, heat through a minute or two and serve (see options below).
Note: If you don’t want to use wine in the jus, simply replace with an equal amount of beef stock. Because roasts can vary in thickness, absolutely always use an instant-read meat thermometer to gauge doneness. A rare roast is done at 120˚F; medium-rare will be 125˚F to 130˚F. The mix of fresh and sliced mushrooms one could use in the quinoa pilaf include white, brown, oyster and shiitake mushrooms. If you use oyster mushrooms, be sure to trim off the tough lower part of the stem before slicing. If using shiitake mushrooms, be sure to remove and discard its very tough stem before slicing the cap.