Add 1 Tbsp oil to a large skillet over medium heat.
Add ham and cook for 2-4 minutes , stirring once or twice until slightly browned
Remove ham from the skillet and add to a medium size mixing bowl.
Add remaining oil to the skillet and stir in onions, cooking on medium low heat for about 10-12 minutes until very soft.
Add spinach for 1-2 minutes then transfer onion-spinach mixture to the mixing bowl with ham.
In a separate bowl, whisk together eggs, milk, nutmeg, salt, and pepper.
In a lightly greased 11 x 7" pan, arrange chopped croissants, all of the pieces should be touching each other and perhaps even overlapping slightly.
Top the croissants with the onion / ham / spinach mixture, spreading it around evenly.
Pour the egg mixture on top until the liquid settles evenly around the pan, possibly nudging the croissants in certain places so that the egg mixture can get through.
Top with cheese.
Cover casserole with aluminum foil and chill for at least 2 hours, but up to overnight.
When ready to bake, preheat oven to 375 degrees.
Bake for 45 minutes while covered then remove the foil for an additional 10 minutes to brown the tops. (I let them "brown" for about 25 minutes-- too long!)