Melt butter in a skillet over medium-high heat. Saute mushrooms and onions in hot butter until soft and caramelized, about 5 minutes.
Pour 1/2 cup chicken stock into the skillet and bring to a boil while scraping the browned bits of food off of the bottom of the pan with a wooden spoon.
Gradually stir flour into mushroom mixture; cook and stir until light golden brown, about 30 seconds. Add remaining chicken stock to mushroom mixture; simmer, stirring occasionally, until sauce thickens, about 5 minutes.
Stir yogurt into sauce; add cream cheese and stir until cheese melts and sauce is thickened, about 2 minutes. Stir chicken into sauce and cook until heated through, about 5 minutes more.