1. Combine potatoes, 2 1/2 cups water, and salt in medium saucepan; cover and bring to a boil over high heat. Reduce heat to medium; simmer, covered, 7 minutes. Add peas; cover and cook 5 minutes or until potatoes are tender. Drain potato mixture in a colander over a bowl, reserving 1 cup cooking liquid.
2. Melt butter in saucepan over medium heat. Add onion, sauté 4 minutes or until tender. Add spinach and corn; cook 3 to 4 minutes or until spinach wilts and corn is thoroughly heated. Remove from heat.
3. Pass potatoes and peas through a ricer or food mill or mash with a potato masher. Stir potato mixture and enough of reserved cooking liquid into spinach mixture to create a creamy texture.
Note: If you have leftovers, you can refrigerate them, and then the next day form them into patties and pan-fry.