Coat a large skillet with nonstick spray and bring to high heat. Season roast with 1/4 teaspoon each salt and pepper. Cook and rotate until all sides are browned, about 5 minutes. Place in a slow cooker. Add all remaining ingredients except cornstarch, including 3/4 teaspoon each salt and pepper. Gently stir. Cook on high for 3 to 4 hours or on low for 7 to 8 hours, until roast is cooked through. Turn off slow cooker. In a small bowl, dissolve cornstarch in 2 tablespoons cold water. Stir cornstarch mixture into liquid in slow cooker. Let sit, uncovered, until slightly thickened, about 5 minutes. Discard thyme. Slice meat and top with veggies and sauce.