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 on medium-high heat (375 degrees) about 5 minutes or until onions are tender, stirring frequently.
Remove the stem from the portabella and the black veins using a spoon. cut into steak like strips.
Stir in the mushrooms and garlic. Cook and stir for 5 minutes or until the 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, on medium heat (275 degrees) Cook and stir until thickened and bubbly. Cook and stir for 1 minute more. Pour the sour cream mixture over pasta mixture. Stirring gently to coat. Sprinkle with chives.