In 2-qt saucepan, warm olive oil and cook onion over medium heat for 5 minutes. Add broth and season to taste. Bring to boil. Add potatoes & return to boil, then simmer, covered, about 12 minutes.

In a skillet, heat butter and cook mushrooms over medium heat until liquid has evaporated. Reserve.

Remove saucepan from heat and blend (or run through food mill) onion-potato mixture. Add spinach and arugula. Correct seasoning. Top with mushrooms and parsley and serve immediately.