Heat a large cast iron skillet over medium heat and add the butter. When the butter is melted, add the onion, carrot, and celery. Sauté until mostly tender, about 8 minutes.
Stir in the garlic powder, thyme, salt and pepper. Cook, stirring often, then add the flour and cook stirring often for about 2-3 minutes.
Slowly stir in the milk, then the broth. Bring to a simmer, stirring occasionally, for about 5 minutes, until thickened.
Stir in the chicken and peas. Simmer for another couple of minutes, then remove from the heat. Top with enough tater tots to cover the top of the filling.
Transfer to the oven and bake for 30-40 mins or until the tater tots are golden brown and the edges are bubbling.