Beef Tenderloin With Shallot Mustard Sauce

INGREDIENTS

• 1 1/2 pounds beef tenderloin, trimmed and tied
• 2 teaspoons coarse salt
• 1 teaspoon freshly ground pepper
• 1 tablespoon olive oil
• 8 ounces shallots (3 large), peeled and cut into 1/4-inch-thick rounds
• 2 tablespoons balsamic vinegar
• 1 cup homemade beef or chicken stock, or canned
• 1 tablespoon Dijon mustard
• 2 teaspoons grainy mustard
• 2 tablespoons unsalted butter