01 - Preheat oven to 375°F. Lightly grease a 9x13-inch casserole dish.
02 - Cook wild rice blend according to package instructions using vegetable broth or water. Set aside.
03 - Heat olive oil in a large skillet over medium heat. Cook onion until translucent, about 3 to 4 minutes.
04 - Add garlic, mushrooms, celery, and carrots. Sauté until mushrooms are tender and vegetables soften, approximately 8 to 10 minutes.
05 - Stir in thyme, salt, and black pepper. Cook for an additional minute, then remove from heat.
06 - In a medium saucepan, melt butter over medium heat. Whisk in flour and cook for 1 minute.
07 - Gradually whisk in milk and heavy cream. Stir continuously until sauce thickens, about 3 to 4 minutes.
08 - Stir in Parmesan cheese and soy sauce. Remove from heat.
09 - In a large bowl, mix cooked rice, sautéed vegetables, and creamy sauce thoroughly.
10 - Spread mixture evenly in the prepared casserole dish.
11 - Combine panko breadcrumbs, melted butter, and parsley in a small bowl. Evenly sprinkle over the casserole. Optionally, add shredded Gruyère or Swiss cheese on top.
12 - Bake uncovered for 25 to 30 minutes until the topping is golden brown and casserole is bubbling.
13 - Allow casserole to cool for 5 minutes before serving.