Add quinoa to the pot, followed by the chicken or vegetable broth, salt, dry mustard and pepper. Bring to a boil then reduce heat and simmer for 15-20 minutes until all liquid has been absorbed. Stir well.
Now, add the grated cheddar cheese and milk. Stir to combine then pour into a greased 9 x 13" casserole dish. In a small bowl, combine the Panko and the mozzarella cheese. Sprinkle on top of casserole and bake for about 30 minutes until golden.