You know this. Sautee onions in butter. Add fresh spinach and wilt down, or add thawed frozen spinach with its liquid as well. Add stock or water. Cook a bit. Blitz in blender or use a stick blender. Then add cream, half and half, whatever you have.
My only suggestion would be to add grated nutmeg somewhere along the line. Not too much, maybe 1/4 t for 4 to 5 cups of soup?