Preheat the oven to 375°F (190°C).

Melt 3 tablespoons of butter in a large skillet over medium heat. Add the diced onion, carrots, celery, and minced garlic; cook until softened, about 5-7 minutes.

Stir in fresh thyme, kosher salt, and black pepper. Sprinkle the flour over mixture and stir to combine.

Gradually add chicken broth and milk, stirring constantly until the mixture thickens, around 3-5 minutes.

Mix in cooked chicken and frozen peas, combining everything well.

Transfer the mixture to a greased casserole dish, spreading it evenly.

Place the biscuits on top of the chicken mixture, spreading them to cover as much surface area as possible.

Brush the remaining melted butter over the biscuits.

Bake for 25-30 minutes until the biscuits are golden brown and cooked through.