Poppy Seed Chicken

Name: Poppy Seed Chicken
Ingredients: 7 tablespoons unsalted butter, plus more for greasing; 2 large yellow onions, finely chopped; Kosher salt; Freshly ground pepper; 1 pound cremini mushrooms, stemmed and sliced 1/4 inch thick; 2 1/4 cups low-sodium chicken broth; 1 1/4 cup milk; 1/4 cup all-purpose flour; 1/2 cup sour cream; 3 tablespoons chopped parsley; 2 teaspoons Dijon mustard; One 1 1/2-pound rotisserie chicken, skin and bones discarded, meat pulled into large bite-size pieces; 2 cups crumbled Ritz crackers (from 2 sleeves of crackers, about 60 crackers); 4 teaspoons poppy seeds