Cook noodles according to package directions. Meanwhile, in a large nonstick skillet over medium heat, cook the mushrooms and shallots in butter for 6-8 minutes or until tender. Add garlic; cook 1 minute longer.
Stir in 1-1/4 cups broth, mustard, salt and pepper. Bring to a boil. Reduce heat; simmer, uncovered, for 10 minutes, stirring occasionally.
Combine flour with remaining broth until smooth; gradually stir into mushroom mixture. Bring to a boil; cook and stir for 2 minutes or until thickened and bubbly. Reduce heat to low; gradually stir in sour cream (do not boil). Drain noodles; serve with mushroom sauce. Sprinkle with parsley.