01 - Preheat oven to 375°F and grease a 9x13-inch baking dish.
02 - Combine wild rice, vegetable broth, and water in a medium saucepan. Bring to a boil, then reduce heat, cover, and simmer for 40–45 minutes until rice is tender and liquid is absorbed.
03 - Heat butter and olive oil in a large skillet over medium heat. Add onion, garlic, mushrooms, celery, and carrots. Sauté for 8–10 minutes until softened and mushrooms release their juices.
04 - Sprinkle flour over the vegetables, stir to coat evenly, and cook for 1 minute.
05 - Gradually whisk in milk, stirring constantly until a thick, creamy sauce forms, about 3–4 minutes.
06 - Remove from heat. Stir in sour cream, Parmesan, thyme, sage, salt, pepper, cooked wild rice, and frozen peas until well combined.
07 - Transfer mixture to the prepared baking dish and smooth the top evenly.
08 - In a small bowl, combine panko, melted butter, and Parmesan. Sprinkle evenly over the casserole.
09 - Bake uncovered for 25–30 minutes until the topping is golden brown and the casserole is bubbling.
10 - Let the casserole rest for 10 minutes before serving to allow flavors to settle.