Preheat the oven to 350 degrees F. Lightly grease a 9x9” baking dish or similar size. Set aside.
Place the chicken pieces into a zip-top bag or a medium bowl. Sprinkle the flour, 2 teaspoons of salt, and pepper, to taste over the chicken. Seal the bag well and shake, or toss in the bowl to completely coat the chicken pieces.
Heat ½ tablespoon of the oil in a medium pan or skillet (a nonstick pan is best) over medium-high heat. Once the oil is hot, add the chicken pieces and brown the chicken. You will most likely have to do this in batches. Use another ½ tablespoon of oil for the next batch of chicken. Once browned, move the chicken to the prepared baking dish. You’re not trying to cook the chicken thoroughly because it will finish cooking in the oven.
Add another 1 teaspoon of the oil to the pan. Add the leeks. Sauté until the leeks are beginning to soften. Add a pinch of seasoned salt. Add the garlic and stir for about 20 to 30 seconds. Pour the mixture over the chicken pieces in the baking dish.
Add the remaining 2 teaspoons of oil to the pan. Add the mushrooms, a pinch of salt, and stir often until nicely browned. Add them to the top of the leeks and chicken.
Maintaining the heat, add the butter to the pan. Once melted, add the flour, stirring constantly until no lumps of flour remain. Continue whisking for another minute. Turn the heat down slightly if the butter is beginning to darken.
While whisking the flour-butter mixture, slowly add the chicken stock to combine. Whisk until smooth, with no lumps of flour remaining. Allow it to come to a gentle boil and thicken.
Turn the heat off and add the milk, stirring to combine. Add the Greek yogurt, whisking until smooth. Taste for seasoning and adjust as desired. Pour over the chicken and mushrooms.
Bake uncovered in the oven for 35 to 40 minutes, or until the chicken chunks are cooked through, a digital thermometer reads 165 degrees F. The amount of cooking time will depend upon the size of your chicken.
Remove from the oven, top with the parsley, and serve warm.