Preheat your oven to 375°F (190°C). Lightly grease a 13x9-inch baking dish.

In a medium saucepan, combine the rice and chicken broth. Bring to a boil over high heat, then reduce to low, cover, and cook for 15 minutes. The rice should be just underdone—still firm in the center. Remove from heat and let sit covered for 5 minutes. Fluff with a fork.

While the rice cooks, melt the butter in a large skillet over medium heat. Add the diced onion and cook, stirring occasionally, until softened and translucent, about 5 minutes. Add the garlic and cook for 30 seconds until fragrant.

Sprinkle the flour over the onion mixture and stir constantly for 1 minute—this cooks out the raw flour taste. Slowly pour in the milk, whisking continuously to prevent lumps. Bring to a gentle simmer; it will thicken in about 2 minutes. Whisk in ½ teaspoon salt, the pepper, paprika, and ground mustard.

Remove the sauce from heat. Stir in 1 ½ cups of the shredded cheddar until melted and smooth. The sauce should be thick enough to coat the back of a spoon.

In a large mixing bowl, combine the cooked rice, cooked chicken, the cheese sauce, and frozen peas (if using). Fold gently until everything is evenly coated. Season with the remaining ½ teaspoon salt, or to taste.

Transfer the mixture to the prepared baking dish, spreading it into an even layer. Sprinkle the remaining ½ cup of cheddar on top.

Bake uncovered for 20-25 minutes, until the edges are bubbly and the top is golden brown. For a deeper golden crust, switch to broil for the last 2 minutes—watch closely so it doesn't burn.

Let the casserole rest for 5 minutes before serving. Garnish with fresh parsley. The internal temperature of the casserole should reach 165°F (74°C) if you're reheating leftovers.