Preheat your oven to 375°F (190°C).

In a large pan, melt the butter over medium heat. Add the chopped onion and cook for about 5 minutes, stirring frequently until the onion becomes translucent.

Add the frozen mixed vegetables directly from the freezer. Then, mix in the remaining cooked chicken, typically around 3 cups. Adjust the quantity as needed based on your preference.

Pour in the chicken broth and season the mixture with dried thyme and seasoned salt.

Prepare a slurry by combining 3/4 cup of milk and 3 teaspoons of cornstarch. This will act as a cream of chicken alternative.

Heat the chicken and vegetable mixture until it comes to a boil. Then, stir in the cornstarch slurry and heat until it thickens and returns to a boil.

Transfer the mixture into a 9×13-inch casserole dish. It should have a thick, creamy consistency, reminiscent of traditional chicken pot pie filling.

To prepare the biscuit-like topping, mix one egg, 1/2 cup of milk, biscuit mix, and 2 tablespoons of melted butter. The result will be a slightly thin batter.

Spoon dollops of the topping over the casserole. As the topping is somewhat thin, it’s best to place it in small amounts across the casserole.

Bake the casserole in the preheated oven for approximately 30 minutes, or until the topping is cooked through and turns golden brown.