Cook egg noodles according to package directions in a large pot of boiling water. Drain.
Trim steak and cut into 2x¼-inch strips.
Melt butter in large skillet over medium high heat. Add meat slices and sauté until meat is seared but is still pink in center. Remove meat slices to a platter with a slotted spoon.
Add onions, garlic and mushrooms to the skillet and sauté until onions are soft and mushrooms just begin to brown. Transfer to platter with slotted spoon.
Remove skillet from heat and deglaze with addition of balsamic vinegar.
Blend in cream cheese and water. Return skillet to stove and heat over medium-low heat, stirring constantly until the sauce is smooth.
Stir in salt and pepper.
Return meat slices and onion/ mushroom mixture to the pan. Continue to heat, stirring and coating meat and vegetables with sauce, until sauce is bubbly.
Serve over warm noodles garnished with chopped chives.