1. For the mushroom sauce, heat 1 tablespoon butter in large skillet over medium-high heat. Add shallots; cook 1 minute until softened. Add mushrooms and 1/4 teaspoon salt; cover and cook 3 minutes. Uncover; cook 2 minutes more until tender. Add wine; boil 1 minute. Add cream; boil 2 to 3 minutes, until slightly thickened. Add broth and water; bring to boil and cook 3 to 4 minutes more.
2. Arrange oven racks in upper third and middle of oven. Heat oven to 450 degree F.
3. For the tenderloin, combine cracked pepper, thyme, and 1 teaspoon salt in cup. Pat beef dry with paper towels; place on large heavy-duty jelly-roll pan. Rub 1 teaspoon butter over beef. Sprinkle evenly with pepper mixture.
4. For the roasted potatoes, brush another large jelly-roll pan with olive oil; add potatoes in single layer. Dot with 1 tablespoon butter; sprinkle with 1/2 teaspoon salt and 1/8 teaspoon pepper.
5. Place roast on the upper rack and potatoes on the lower rack of oven. Roast tenderloin 35 to 40 minutes, until an instant-read thermometer inserted in center of roast registers 140 degree F. Roast potatoes 35 minutes.
6. Remove roast from oven. Tightly cover pan with foil; let stand 10 to 15 minutes (temperature should then register 145 degree F for medium-rare)
7. Meanwhile, switch potatoes to upper oven rack and roast 5 to 10 minutes more, until crisp on outside and tender inside. Spoon potatoes with metal spatula onto large serving platter.
8. Transfer roast to large cutting board; reserve drippings in pan. Cut roast into 1/2-inch-thick slices. Add 1/4 cup hot mushroom sauce to drippings and heat over low heat, stirring to dissolve browned bits. Then stir drippings mixture into remaining mushroom sauce. Divide beef and potatoes among 8 serving plates. Serve with mushroom sauce if desired. Makes 8 servings.