Preheat the oven to 350.
Spray a 9 x 9 (or similar size) baking dish with cooking spray and set aside.
In a large skillet, melt the butter.
Whisk in the flour, and cook 2-3 minutes, until golden brown and bubbling.
Slowly whisk in the chicken broth, stirring constantly to prevent lumps.
Let the sauce thicken slightly, then slowly whisk in the milk, a little at a time.
Bring the mixture nearly to a simmer, and let thicken.
Stir in the salt and pepper.
Remove from the heat
Stir in the sharp cheddar cheese and dijon mustard, until melted.
Return to the heat
Stir in the cooked chicken and cooked tortellini, until heated throughout.
Pour into the prepared baking dish.
In a small bowl, combine the melted butter, both kinds of bread crumbs, and the parmesan cheese.
Sprinkle the bread crumb mixture over the tortellini.
Bake 20-25 minutes, or until the bread crumbs are golden brown and the sauce is bubbling.