Heat oven to 425 degrees F.
Combine olive oil, garlic, salt, and pepper. Press evenly into surface of beef. Heat large nonstick skillet over medium-high heat until hot. Brown beef evenly on all sides to sear. Transfer to rack in shallow roasting pan. Insert meat thermometer so tip is centered in thickest part of roast, not resting in fat. Do not add water or cover. Roast in 425-degree F. oven about 30 to 35 minutes for medium-rare to medium doneness.
Meanwhile, in the same skillet, heat 1-1/2 tablespoons butter over medium-high heat until hot. Add half of mushrooms and the shallots. Cook and stir until mushrooms are tender. Remove from skillet. Repeat with remaining butter and mushrooms. Set aside.
In same skillet, bring port wine to a boil over high heat. Cook 5 to 7 minutes or until almost syrupy (reduced to about 2 tablespoons). Stir in broth, mustard, and cornstarch mixture. Cook until slightly thickened and bubbly, stirring occasionally. Return mushrooms to sauce.
Remove roast from oven when meat thermometer registers 135°F degrees. Transfer roast to carving board. Tent loosely with aluminum foil. Let stand 15 minutes. (Temperature will continue to rise about 10 degrees to reach 145 degrees F. for medium rare.)
Carve roast into slices. If desired, immediately before serving, stir 1 tablespoon softened butter into sauce. Serve with roast.