Preheat the oven to 350°F. Lightly grease an 8x11-inch baking dish.
Melt 6 tablespoons butter in a medium saucepan over medium heat. Add the flour and whisk until a thick, smooth paste forms. Continue whisking for about 2 minutes.
Slowly pour in the chicken broth and milk, whisking continuously. Add the onion powder and cook for 3–5 minutes, stirring often, until the sauce has thickened. Remove from the heat.
In a large bowl, combine the shredded chicken, Greek yogurt, cream sauce, 2 teaspoons poppyseeds, salt, and pepper. Stir until evenly combined.
Spread the chicken mixture evenly into the prepared baking dish.
Place the butter crackers in a large zip-top bag, press out the air, and seal. Using your hands or a rolling pin, crush the crackers into fine pieces. Pour the melted butter and remaining 1 teaspoon poppy seeds directly into the bag, then gently shake and mix until the crackers are evenly coated. Sprinkle evenly over the chicken mixture.
Bake uncovered for 20–25 minutes, or until the casserole is hot and bubbly and the cracker topping is lightly golden.
Let rest for about 5 minutes before serving. Garnish with parsley, if desired.