Cook fettuccine according to package directions. Drain; keep warm. In a small bowl stir together the sour cream and flour. Stir in the water, bouillon, and pepper. Set aside.
In a large skillet cook onions in hot butter about 5 minutes or until onions are tender, stirring frequently. Stir in mushrooms and garlic. Cook and stir about 5 minutes more or until vegetables are tender. Remove mushroom mixture from skillet and add to drained pasta; keep warm.
Wipe out skillet. Stir the sour cream mixture into the skillet. Cook and stir until thickened and bubbly. Cook and stir for 1 minute more. Pour the sour cream mixture over the pasta and mushroom mixture, stirring gently to coat. To serve, sprinkle with chives. Makes 4 servings.