Preheat oven to 375.
In a large saucepot over medium high heat, add the oil. Once hot, add the chopped bell pepper, scallions and spinach and saute for about four minutes—just until bell pepper has started to soften. Add the garlic and continue sauteing for another 30 seconds.
Add quinoa to the pot, followed by the chicken 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 mozzerella cheese. Sprinkle on top of casserole and bake for about 30 minutes until golden.
Let cool for 10 minutes before serving (or not).