Saute: Position a rack in the center of the oven; preheat the oven to 350ºF.  Add a swish of oil to a large saute pan and saute the mushrooms over medium-high heat for  5-7 minutes, remove to a plate.

Sauce: Add 3 tablespoons of butter to the same saute pan, once it melts add the flour and whisk. Cook for 1-2 minutes to remove the raw flour taste. Slowly stream in the chicken broth about 1/4 cup at a time while you whisk using your other hand. Allow the sauce to start bubbling, then let thicken for 3-4 minutes or until it just coats the back of a spoon. Lower the heat to the lowest heat setting and add the sour cream, 1 teaspoon garlic powder, and onion powder as you whisk to make a smooth sauce. If the heat is too high the sour cream will curdle (fine to eat, just not as visually appealing.) Once the sour cream is mixed in, stream in the cream. Kick the heat back up to medium-high and allow the sauce to thicken, 2 minutes.

Assemble: Remove from heat, stir in the cheddar cheese and let it melt. Taste and adjust with salt and pepper as desired. Fold in the noodles, sauteed mushrooms, and shredded chicken. Spray a 9x13 baking pan with nonstick cooking spray, add the prepared tetrazzini to the pan. Melt the remaining 3 tablespoons of butter in a small bowl, stir in the parmesan, Italian breadcrumbs, and the remaining ½ teaspoon of garlic powder (if desired.) Scatter the breadcrumb mixture evenly over the casserole. Bake, uncovered for 30-35 minutes or until the breadcrumbs brown and the sauce bubbles around the rim. Brown under the broiler for a few seconds to darken the breadcrumbs more if you'd like. Serve warm topped with chopped parsley if desired.