Wash the spinach and cut off and discard the root ends and any tough stems. Chop the spinach coarsely and add to the onions. Cover and cook over medium heat for ten minutes.
Sauté the mushrooms in the lemon juice and remaining butter. Add to the spinach along with the seasonings. Toss lightly. If desired, garnish with additional sautéed sliced mushrooms. Serve immediately.