Preheat oven to 350°F. Rinse rice in a fine-mesh sieve under cold water for 2 to 3 minutes, or until the water runs clear. Set aside. Bring 12 cups water to a boil in a large pot. Once boiling, add 1 tablespoon salt and the chopped broccoli. Boil for 3 minutes, then use a slotted spoon to transfer broccoli to paper towels. Let dry completely.
Return the water to a full boil, then add the rinsed rice. Cook, stirring occasionally, for exactly 10 minutes. Do not reduce the heat. Taste to make sure rice is tender; if needed, cook 1 more minute. Drain rice in a fine-mesh sieve and let stand for 5 minutes to dry. Spread rice in an even layer in the bottom of a 9x13-inch casserole dish. Add broccoli on top once it is completely dry.
Chop the chicken to get 3 cups. Add evenly over the broccoli and rice
Sauce: Melt butter in a small pot over medium heat. Gradually whisk in flour to form a thick paste. Whisk constantly for 1 minute. Slowly pour in 1/2 cup milk, then 1/2 cup chicken broth, whisking constantly. Add another 1/2 cup milk, the remaining 1/2 cup broth, and the remaining 1/2 cup milk. Whisk until smooth and nicely thickened. Remove from heat. Add 1 and 1/2 cups cheese, a small handful at a time, stirring until melted and smooth. Add sour cream and whisk until smooth. Stir in chicken bouillon powder, Dijon mustard, garlic powder, onion powder, dried parsley, and salt and pepper to taste. I add 1/4 to 1/2 teaspoon salt and 1/2 teaspoon pepper. Stir in poppy seeds.
Pour sauce evenly over the casserole and spread to coat. Sprinkle remaining 1/2 cup cheese over the sauce. Melt 2 tablespoons butter in a small saucepan over medium-high heat. Add panko and cook, stirring constantly, until lightly browned. Sprinkle toasted panko evenly over the cheese. Cover casserole with foil and bake for 30 minutes, or until bubbly around the edges. Serve hot.
Notes
Note 1: I use 1% milk for a lighter sauce. Use 2% or whole milk for a creamier sauce. Avoid skim or plant-based milk.Note 2: Dijon adds a little tang. Reduce or skip it if you’re not a fan.Note 3: Crushed Ritz crackers also work. Panko is crunchier than regular breadcrumbs and is usually near the breadcrumbs or in the Asian food section.Storage: Store leftovers in an airtight container in the fridge for up to 3 days. Reheat: Warm in a 350°F oven or microwave individual portions in short bursts. Freeze for up to 2 months, then thaw in the fridge before reheating.