01 - Preheat oven to 375°F. Grease a 9x13-inch baking dish.
02 - Combine wild rice blend, vegetable broth, and salt in a medium saucepan. Bring to a boil, then reduce heat, cover, and simmer for 40 to 45 minutes until rice is tender. Drain any excess liquid.
03 - Heat olive oil in a large skillet over medium heat. Add onion and sauté for 3 minutes. Stir in garlic, mushrooms, celery, and carrot. Cook 8 to 10 minutes until vegetables are tender and mushrooms browned. Add thyme and black pepper, cook for 1 additional minute. Remove from heat.
04 - Melt butter in a separate saucepan over medium heat. Whisk in flour and cook for 1 minute. Gradually whisk in milk, simmering and stirring constantly until thickened, about 4 to 5 minutes.
05 - Remove from heat and stir in sour cream, Dijon mustard, and Parmesan cheese until smooth.
06 - In a large bowl, mix cooked rice, sautéed vegetables, and cream sauce until well combined. Spread mixture evenly in the prepared baking dish.
07 - Combine panko breadcrumbs with melted butter in a small bowl. Sprinkle evenly over the casserole.
08 - Bake for 25 to 30 minutes until bubbling and topping is golden brown. Let cool for 5 minutes before garnishing with parsley, if desired, and serving warm.