Refrigerate covered, overnight or at least 8 hours. Wash spinach and drain well. Put onions, spinach, and broth in a pot with remaining olive oil and bring to a boil. Add rice, stir, add salt and pepper to taste, and allow to simmer for 25 minutes, or until rice has absorbed broth and is tender and fluffy when stirred. Clean mushrooms, keeping caps whole and chopping the stems.
Saute stems in butter and put aside.
Cook and drain spinach. Make a roux of flour and butter. Add milk and stir to make cream sauce. Combine spinach, chopped mushrooms and cream sauce, and season to taste. Spoon creamed spinach over artichoke bottoms.