Preheat oven to 425°F. Pat the cross rib roast dry then place the roast into a lidded roasting pan or Dutch oven. Make sure that the roaster or Dutch oven is large enough to add a lot of vegetables around the roast later!
In a bowl, mix the butter, garlic, rosemary, thyme, salt, and pepper into a paste. Spread the paste over the top and sides of the roast.
Roast uncovered in the preheated oven for 20 minutes, letting the high heat brown the roast all over. Turn down the oven temperature to 325 °F.
Whisk together the gravy ingredients. Pour around the sides of the beef roast. Place the lid on top and roast for another 3 hours.
After 3 hours, remove the roast from the oven and add the vegetables around the meat in the gravy. Place the lid back on and return to the oven. Cook for another 1-2 hours, until the vegetables are soft and the meat is fall-apart tender.
Remove the roast and vegetables to a platter and cover to keep warm while you prepare the gravy.
Gravy
To make the gravy, take 1/4 cup cold water and 2 tablespoons of cornstarch and whisk together until combined.
Remove the drippings from the pan and spoon into a saucepan. Scrape up and dissolve any brown flavor bits from the bottom of the pan into the liquid while removing. If you used a Dutch oven, simply place the Dutch oven full of drippings on the stovetop.
Whisk the cornstarch slurry into the drippings quickly.
Heat, over medium-high heat, whisking continuously, until the gravy has thickened. Remove from heat and place in a gravy bowl.
To serve, slice the roast and serve with the vegetables, pouring the gravy on top.