Meanwhile, combine the cornstarch and the remaining ¼ cup water in a small bowl, stirring to dissolve the cornstarch. Set aside. Place the roasting pan across two burners over medium-high heat. Add the stock and stir to scrape up any browned bits from the bottom of the pan. Using a wooden spoon, mash garlic into the juices and stir. Using a slotted spoon, transfer the vegetables from the pan into a strainer set over a small saucepan. Pour the juices from the roasting pan over the vegetables and press firmly on them with the back of a spoon. Set the saucepan over medium-high heat, bring to a simmer and stir in the cornstarch mixture. Cook until thickened, about one minute. Season with kosher salt and pepper to taste. Serve with the gravy.