Preheat the oven to 375°F (190°C).
In a large skillet, melt the butter over medium heat. Add the diced onion and cook for 3-4 minutes until softened.
Add the minced garlic and sliced mushrooms to the skillet and cook for another 4-5 minutes until mushrooms are tender.
Sprinkle the flour over the vegetables in the skillet and stir well to combine. Cook for 1-2 minutes to remove the raw flour taste.
Gradually whisk in the chicken broth and milk, stirring constantly until the sauce thickens and comes to a simmer.
Add the dried thyme, dried sage, salt, and black pepper to the sauce. Stir well.
Remove the skillet from heat and stir in the shredded turkey, thawed mixed vegetables, and cooked rice until evenly combined.
Transfer the mixture into a 9×13 inch baking dish and spread evenly.
Sprinkle the shredded cheddar cheese evenly over the top of the casserole.
In a small bowl, mix the breadcrumbs with olive oil until well coated. Sprinkle the breadcrumb mixture evenly over the cheese layer.
Bake the casserole in the preheated oven for 30-35 minutes or until the topping is golden brown and the casserole is bubbling.
Remove from the oven and let it rest for 5 minutes before serving.