Preheat oven to 350°F. Grease a 9×13-inch casserole dish with butter or olive oil spray and set aside.
Add cooked millet to a large mixing bowl.
In a large skillet, heat half the butter/oil over medium-high. Add onions, carrots, and celery; cook until tender.
Stir in garlic, sage, parsley, and poultry seasoning; cook 2–3 minutes more until fragrant. Add the mixture to the millet.
In the same skillet, heat the remaining butter/oil and sauté mushrooms with thyme until golden. Add to the bowl.
Mix everything together and season with salt and pepper.
Transfer to a baking dish and bake 25–35 minutes until hot through and golden on top.