Preheat oven to 350°. Prepare a 9x13 casserole dish by coating with cooking spray.
In a large deep skillet or tall stock pot, melt 4 tablespoons butter over medium heat. Add onions, carrots, celery and mushrooms then cook until softened. Add salt and pepper.
Sprinkle vegetables with flour and stir until they are coated. Slowly whisk in broth and increase heat to medium-high. Bring to a low boil and continue stirring until thickened, about 4-5 minutes. Turn heat off then add cream cheese and 3/4 cup parmesan. Stir until cheese is melted.
Add herbs, cooked spaghetti, turkey, and peas to the sauce then stir to combine. Pour into casserole dish.
Melt remaining 2 tablespoons butter and pour into a small bowl with Panko and remaining 1/4 cup parmesan. Mix together then sprinkle over top of the casserole.
Bake uncovered for 30-40 minutes or until bubbly and lightly browned on top.