Preheat your oven to 375°F and lightly grease a 9x13 inch baking dish.
Heat the olive oil in a large skillet over medium heat, then sauté the chopped onion until it becomes soft and translucent, about 5 minutes.
Add the minced garlic to the skillet and cook for 1 more minute until fragrant, then remove the skillet from the heat.
In a large mixing bowl, combine the sautéed onion and garlic with the shredded chicken, thawed peas and carrots, cream of chicken soup, sour cream, dried thyme, black pepper, and salt, stirring until everything is evenly incorporated.
Spread the chicken mixture evenly into the prepared baking dish.
Arrange the frozen tater tots in a single layer over the top of the chicken mixture, covering it completely.
Bake for 35 to 40 minutes, or until the casserole is bubbling around the edges and the tater tots are golden brown and crisp.
Let the casserole stand for 5 minutes before serving to allow it to set slightly.