Cook pasta to desired doneness as directed on package.
Meanwhile, spray 12-inch nonstick skillet with nonstick cooking spray. Heat over medium-high heat until hot. Add beef and garlic; cook about 4 minutes, stirring frequently. Add mushrooms, broth and onions. Reduce heat; cover and cook 3 minutes. Uncover; cook an additional 2 to 3 minutes.
In small bowl, combine flour and milk; blend until smooth. Add to skillet; cook and stir 2 minutes or until slightly thickened. Stir in mustard until smooth.
Drain pasta; arrange on serving platter. Spoon beef-mushroom mixture and sauce over pasta. Sprinkle with pepper.