Step 1: Cook the Wild Rice : If not using pre-cooked rice, cook wild rice according to package instructions (usually 45–50 minutes). Fluff with a fork and set aside.
Step 2: Sauté the Vegetables : Preheat oven to 375°F (190°C). In a large skillet, heat olive oil or butter over medium heat. Add onion, carrots, and celery. Sauté for 4–5 minutes until softened. Add garlic and mushrooms (if using). Cook for another 3 minutes. Stir in thyme, sage, salt, and pepper. Remove from heat.
Step 3: Make the Creamy Sauce : In a medium saucepan, melt butter over medium heat. Whisk in flour to make a roux; cook for 1 minute. Gradually whisk in broth and milk. Continue whisking until thickened, about 3–5 minutes. Stir in sour cream (or Greek yogurt) and cheese (if using). Season with salt and pepper. Do not boil after adding sour cream.
Step 4: Assemble the Casserole : In a large mixing bowl, combine cooked rice, shredded turkey, sautéed veggies, and creamy sauce. Stir gently until everything is evenly coated. Transfer mixture to a greased 9x13-inch baking dish.
Step 5: Add Topping : In a small bowl, mix breadcrumbs with melted butter and Parmesan. Sprinkle evenly over the casserole.
Step 6: Bake : Bake uncovered for 25–30 minutes, or until the topping is golden and the casserole is bubbling around the edges. Let rest for 5 minutes before serving.