01 - Preheat oven to 375°F. Lightly grease a 9x13-inch baking dish.
02 - Prepare the wild rice blend according to package directions using vegetable broth or water; set aside once cooked.
03 - Heat olive oil in a large skillet over medium heat. Add chopped onion and cook until translucent, about 3–4 minutes.
04 - Stir in garlic, mushrooms, celery, and carrots. Sauté until mushrooms are tender and vegetables soften, approximately 8–10 minutes.
05 - Add thyme, salt, and black pepper; cook for 1 more minute then remove from heat.
06 - In a medium saucepan, melt butter over medium heat. Whisk in flour and cook for 1 minute to form a roux.
07 - Gradually whisk in milk and cream. Cook, stirring constantly, until sauce thickens, about 3–4 minutes.
08 - Remove from heat and stir in Parmesan cheese and soy sauce until fully incorporated.
09 - In a large bowl, combine cooked wild rice, sautéed vegetables, and creamy sauce. Mix thoroughly.
10 - Spread the mixture evenly in the prepared baking dish.
11 - Mix panko breadcrumbs with melted butter and chopped parsley. Sprinkle evenly over the casserole. Add shredded cheese if desired.
12 - Bake uncovered for 25–30 minutes until the topping is golden and the casserole bubbles.
13 - Allow to cool for 5 minutes before serving.