01 - Preheat your oven to 375°F. Grease a 9x13-inch baking dish with butter or cooking spray.
02 - Combine wild rice, vegetable broth, and water in a medium saucepan. Bring to a boil, then lower 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 vegetables are softened and mushrooms release their juices.
04 - Sprinkle flour over the vegetables, stir to coat evenly, and cook for 1 minute to remove raw flour taste.
05 - Gradually add milk while stirring constantly until a thick, creamy sauce forms, about 3-4 minutes.
06 - Remove from heat. Stir in sour cream, Parmesan, thyme, sage, salt, and pepper. Mix in cooked wild rice and frozen peas until well combined.
07 - Transfer mixture to the prepared baking dish and smooth the top evenly with a spatula.
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 around the edges.
10 - Let rest for 10 minutes before serving to allow the casserole to set for easier scooping.