1 4 1/2-pound prime rib roast of beef (2 ribs), trimmed and tied
1 tablespoon Dijon mustard
1 teaspoon prepared horseradish
1 3/4 tablespoons chopped fresh rosemary
1 1/2 teaspoons kosher salt
1/2 teaspoon freshly ground black pepper
1 large onion, quartered
1 large whole shallot
2 carrots, peeled
2 celery stalks, ends trimmed
6 ounces portobello mushrooms
Olive oil
Herbed Yorkshire Pudding
1 cup all-purpose flour
2 eggs
1 cup whole or low-fat milk
3/4 tablespoon minced chives
2 teaspoons minced fresh rosemary (1 teaspoon dried)
3/4 teaspoon kosher salt
2 tablespoons unsalted butter
2 tablespoons vegetable oil
1 cup Cabernet Sauvignon
1 1/2 cups beef stock
Kosher salt and freshly ground black pepper
1/2 teaspoon cornstarch mixed with 1/2 teaspoon cold water
GARNISH: minced chives