Preheat oven to 375 degrees F (190 degrees C).

Heat oil in a large skillet over medium heat. Add chicken and onion; cook and stir until chicken juices run clear and onion softens, about 5 minutes.

Place cubed potatoes in a pot and cover with water; bring to a boil. Cook for 5 minutes; drain.

Combine chicken mixture, potatoes, thawed vegetables, and gravy in a large bowl. Add onion powder, garlic powder, thyme, salt, and black pepper; stir until well-distributed.

Pour mixture into a 9-inch glass baking dish and arrange potato nuggets in a single layer on top.

Bake casserole in the preheated oven until bubbly around the edges, about 45 minutes.