Cook noodles in a large saucepan according to directions. Drain noodles and clean saucepan to make the sauce.
In a large saucepan over medium heat, melt the butter until bubbly. Add the celery, carrot, and onion and cook for 3 minutes, until vegetables have softened.
Whisk in the flour, salt, pepper, and thyme. Cook, whisking constantly, for 1 minute.
Gradually add chicken broth and milk, whisking constantly. Bring to a boil, then reduce heat and simmer until the sauce begins to thicken.
Stir in the cheese and ham and cook for 1 minute, or until cheese is melted.
Remove the ham mixture from the cooktop and stir in the cooked egg noodles. Transfer the ham and noodle mixture to a prepared 9x13 casserole. Sprinkle with seasoned panko crumbs (if using) and bake in the oven for 30 minutes, or until casserole is hot.