Heat 1 tablespoon oil in a Dutch oven over medium heat.
Brown beef with garlic in batches; pour off drippings.
Return beef to pan; season with salt and pepper. Stir in broth and thyme; bring to a boil. Reduce heat; cover tightly and simmer for 2 hours or until beef is fork tender.
Meanwhile, heat oven to 425°F. Line a baking pan with foil and coat foil with cooking spray. Place mushrooms, tomatoes, and onions on prepared pan. Drizzle with remaining oil and 2 1/2 tablespoons vinegar; toss to combine.
Roast for 20 to 25 minutes, or until tender.
Prepare couscous according to package directions.
Combine cornstarch and water until dissolved. Stir into beef. Bring to a boil, stirring constantly, and cook for 1 minute.
Stir in vegetables and remaining vinegar. Serve over couscous.
If you want to start using pearl couscous in your cooking, check out the what is couscous? section of our website.