Green yen choy leaves, stems removed and cut in about 1-inch pieces
portabello mushrooms, diced
Saute mushrooms and garlic in butter. Add yen choy, stir until wilted. Set aside.
In the same pan, add flour and equal amount of butter to make a roux (approximately 1/2 tablespoon each for a sauce serving 2 persons). Slowly add some warmed milk and whisk until thickened. Add yen choy mixture.
Boil fettucine in salted water. Add pasta to creamy yen choy mixture and heat until very hot, mixing constantly. Stir in some cheese. At this point the sauce may get too thick, thin pasta sauce by adding some of the salted water.