I used to make this combination during my bachelor days.
Sometimes I just stirred up mashed potatoes and canned spinach, added butter, salt and pepper and ate it. Other times I grated cheese over it, and stuck in the oven until the cheese started browning lightly.
When I was feeling flush, I'd cut a Polish sausage or two into 1/2" rounds and fry them, along with sliced onion. Then I'd make flour gravy out of the drippings, and stir the onion and sausage slices back into it. I'd pour that into a well in the center of the potatoes and spinach.