Preheat the oven to 375°F (190°C).

In a large skillet or saucepan, melt 4 tablespoons butter over medium heat.

Add the diced onion and carrot. Cook, stirring occasionally, until softened (about 5–7 minutes). Sprinkle 1/2 teaspoon kosher salt over the vegetables.

Sprinkle the 1/4 cup flour over the vegetables and stir for 1–2 minutes to cook out the raw flour taste.

Slowly whisk in the 3 cups whole milk until smooth. Bring to a gentle simmer, stirring constantly until the sauce thickens (about 4–6 minutes).

Stir in 2 teaspoons Better Than Bullion Chicken Base, 1/4 teaspoon garlic powder, 1/2 teaspoon dried thyme, 1/4 teaspoon onion powder, and 1/4 teaspoon black pepper. Taste and adjust seasoning.

Add the chopped rotisserie chicken and frozen peas. Stir to combine and heat through.

Remove from heat and stir in 1/4 cup sour cream and the cooked rice. Taste and add the final 1 teaspoon kosher salt if needed.

Transfer the mixture to a greased 9x13-inch baking dish, spreading it into an even layer.

In a small bowl, mix the crushed Ritz crackers with 2 tablespoons melted butter. Sprinkle evenly over the casserole.

Bake at 375°F for 25–30 minutes, until the filling is bubbly and the cracker topping is golden brown.

Let rest for 5–10 minutes before serving so the casserole sets slightly.