Preheat oven to 350 degrees Fahrenheit. Set rack in the middle.
Fill a large pot with water, add 1 tsp of salt and bring to a boil. Add in the elbow macaroni, reduce heat to medium and cook until almost done. See note.
While the macaroni is boiling, wash your spinach and remove any tough stems. Chop your spinach roughly and add the dill and green onions to it. Set aside.
Begin to prepare your cheese sauce by adding the butter and flour to a large pot. Over medium high heat melt the butter and stir frequently until the flour takes on a pale brown color. Carefully add in the buttermilk, the milk, the paprika, onion powder, salt and pepper.
Cook, stirring regularly, until the sauce begins to thicken; this should take approximately 5 minutes.
Once your sauce has thickened add in the ricotta cheese, the grated graviera and the crumbled feta. Stir until all of the cheese has melted.
Next add in the spinach, dill and green onion mix. If the spinach is still somewhat wet after being washed, that's okay. Reduce heat to medium low and cook, stirring regularly, until the spinach appears wilted in the sauce. Remove from the heat.
Drain your pasta if you have not already done so and add the pasta to the cheese and spinach sauce. Mix well until everything is well combined.
Butter the bottom and sides of a 9 x 11 inch casserole dish. Pour in the macaroni and cheese mixture and using a rubber spatula smooth out the top.
In a small bowl whisk together the bread crumbs and the mizithra cheese and then sprinkle it evenly over the casserole.
Bake, covered, in the middle rack of your oven for 20 minutes. Remove the cover and cook for an additional 10 to 15 minutes until the top is golden brown and the sauce is bubbling.
Remove the mac and cheese from the oven and allow to cool for approximately 5 - 10 minutes before serving.