01 - Preheat oven to 350°F. Grease a 9x13-inch baking dish with butter or cooking spray.
02 - In a medium saucepan, combine wild rice blend, vegetable broth, and water. Bring to a boil, then reduce heat to low, cover, and simmer for 40 to 45 minutes until rice is tender and most liquid is absorbed. Drain any excess liquid.
03 - Heat butter and olive oil in a large skillet over medium heat. Add chopped onion, celery, and carrots; sauté for 5 minutes until softened.
04 - Add minced garlic, sliced mushrooms, thyme, rosemary, salt, and pepper. Cook for 8 to 10 minutes, stirring occasionally, until mushrooms release moisture and begin to brown.
05 - Sprinkle flour evenly over the vegetables and stir thoroughly to coat. Cook for 1 minute to eliminate raw flour taste.
06 - Gradually add whole milk and heavy cream while stirring constantly. Continue cooking until the sauce thickens, about 3 to 4 minutes.
07 - Remove skillet from heat. Stir in sour cream, grated Parmesan, and shredded Gruyère until smooth. Fold in the cooked wild rice until fully incorporated.
08 - Transfer the mixture into the prepared baking dish, spreading evenly.
09 - In a small bowl, mix panko breadcrumbs with melted butter. Sprinkle the mixture evenly over the casserole. Bake uncovered for 25 to 30 minutes until bubbly and golden brown on top.
10 - Allow the casserole to cool for 5 minutes before serving. Garnish with chopped parsley if desired.