Cook pasta according to directions in heavily salted water until 1 minute BEFORE it's al dente.
Meanwhile, sauté the onions in the olive oil (2 tablespoons) in a large pot or skillet over medium-high heat until softened and starting to brown (about 3 minutes)
Add the sliced mushrooms and ½ tsp. salt to the pot and sauté for a few minutes more until most of the liquid has evaporated and mushrooms have developed a golden brown color, stirring infrequently (about 4 minutes).
Add the all-purpose flour (2 tablespoons) and stir to coat each mushroom well, getting rid of as many chunks of flour as possible and cooking out the raw flour taste (about 1 minute).
Add the chicken stock gradually and stir well, incorporating the flour as much as possible. Add the almost al dente pasta and ½ cup of the pasta cooking water. Bring to a simmer to thicken and finish cooking the pasta, stirring frequently (about 2 minutes).
Turn down the heat to low. Add the cooked chicken and Greek yogurt and heat through. Taste and adjust seasoning as necessary. Serve hot.
