Bake bread cubes on baking sheet with a lip in preheated 200° F oven for 30 minutes stirring once.
Meanwhile, heat oil in skillet, sauté onions on medium heat 15 minutes, add ham. Continue to cook until onions are well browned.
Whisk together eggs, milk and seasonings.
Place half of dried cubes into greased 9x13x2-inch pan.
Top with half the spinach, meat mixture, egg mixture then cheese. Repeat.
Cover with foil and refrigerate overnight.
Bake covered in a preheated 325° F oven, 55 minutes.
Remove foil and bake another 10-15 min., or until knife comes out clean when inserted in center.
Let sit 10 minutes before serving.