Note: Ask your butcher for a roast cut from the top portion of the sirloin. These are more tender and sometimes referred to as "spoon roasts." Pat the roast dry with paper towels. Using a sharp paring knife, make slits over the entire surface of the roast and insert the garlic slivers. In a small bowl, mix together the thyme, 1 1/2 teaspoons kosher salt, 1/2 teaspoon pepper and 3 tablespoons of the oil. Brush this mixture on all sides of the roast. (The roast can be prepared 1 day ahead. Cover and refrigerate. Bring to room temperature for 30 minutes before proceeding.)
Arrange one rack at center position in the oven and another at a lower position and heat the oven to 450 degrees.
Lightly oil the bottom of a medium, flameproof roasting pan and stand the roast, fat-side up, in the center of the pan. This particular cut of beef sometimes tips over as it roasts. To prevent this, use a roasting rack with sides that you can adjust to steady the meat. Roast the meat for 15 minutes.
While the meat is roasting, prepare the onions and mushrooms. Oil a large, rimmed baking sheet generously. Peel the onions and cut them into wedges 3/4 inch thick, leaving the root ends intact. Arrange the onions on half of the baking sheet and the mushrooms on the other half and drizzle both with the remaining 1/2 cup olive oil. Toss the vegetables lightly to coat well, adding more oil if necessary. Salt and pepper the vegetables.
After the meat has roasted for 15 minutes, reduce the heat to 350 degrees and place the pan with the vegetables on the lower shelf. Continue to roast the beef until an instant-read thermometer inserted into the center of the meat registers 130 to 135 degrees, about 50 to 60 minutes. Roast the vegetables, stirring every 15 minutes, until slightly browned and charred around the edges, for 50 to 60 minutes.
When done, transfer the roast to a cutting board and let rest for 20 minutes. If the vegetables are not done when the roast is, continue roasting a few minutes more, checking every 5 minutes, until done. Remove the vegetables and tent them with foil to keep warm.
Skim off and discard any fat in the roasting pan. Place the pan over medium-high heat, and add the broth and wine. With a wire whisk, scrape up any browned bits on the bottom of the pan into the liquids. Bring the mixture to a simmer, and reduce by half. Then swirl in the butter. Season with salt and pepper.
Cut the roast, crosswise against the grain, into slices 1/4 inch thick and arrange on a serving platter. Garnish the platter with bouquets of watercress and surround the meat with the onions and mushrooms. Drizzle the sliced meat with some sauce and pass extra sauce separately. Top each serving with a thin slice of blue cheese (optional).
Leftover tip: Sliced leftover beef is delicious used in sandwiches made with good peasant bread or a baguette. Slather some Dijon mustard on the bread, top with leftover roast beef, crumble any remaining blue cheese over, and add some watercress sprigs or a few arugula or baby spinach leaves plus some sliced tomatoes. Source: " Sunday Roasts " by Betty Rosbottom (Chronicle Books, 2011).