Oven: Preheat oven to 350 degrees F. Lightly spray a 9×13 baking dish with nonstick cooking spray. Prep chicken: While the chopped chicken is still on the cutting board, pat dry with a paper towel. Toss with 1/4 teaspoon salt, 1/4 teaspoon pepper, 1/4 teaspoon paprika. Set aside.
Cook chicken: Heat 2 tablespoons olive oil in a large Dutch oven over medium-high heat. Add chicken in a single layer and sear for one minute without moving chicken, then continue to cook and stir chicken until opaque (it will not be cooked through). Transfer chicken to a large bowl using a slotted spoon. Sauté vegetables and aromatics: Add a drizzle of oil to the drippings if they look dry (or butter for decadence). Add onions, carrots and celery and sauté until onions are tender. Add the garlic and sauté 30 seconds; transfer to the chicken bowl using a slotted spoon. Caramelize mushrooms: Melt 2 tablespoons butter with a drizzle of olive oil over medium high heat in the leftover drippings. Add the mushrooms and give them a stir to evenly coat, then arrange in a single layer. Cook each side until deeply golden. Transfer to the chicken bowl using a slotted spoon.
Cream Sauce: Whisk the chicken broth and cornstarch together in a liquid measuring cup or bowl; set aside. To the now empty pot (don’t wipe out), melt 2 tablespoons butter with 1 tablespoon olive oil over medium heat. Whisk in flour and cook, while whisking, for 2 minutes. Reduce heat to low, then gradually whisk in chicken broth/cornstarch and milk, followed by Dijon, Worcestershire sauce, chicken bouillon and all spices/seasonings.
Bring sauce to a simmer, whisking often, until thickened. Remove from heat and whisk in 1 cup cheddar until melted followed by Parmesan cheese until melted. Combine: Add the chicken/mushroom/veggies and COOKED wild rice blend to the sauce and stir until evenly combined. Taste and seasone with salt/pepper to taste. Transfer to the prepared casserole dish. Top with remaining 1 cup cheddar cheese.
Panko topping: Melt butter in olive oil over medium heat in a medium skillet. Add panko and stir to coat. Continue cooking until crumbs become golden brown. Evenly sprinkle over cheese.
Bake: Cover the casserole and bake at 350 degrees for 30-35 minutes or until hot and bubbly and cheese is melted.