Place potatoes in 5 cups in water and boil for 25 minutes.
After 25 minutes add the watercress, and cook for an additional 2 minutes.
Strain the water from the pot and place the potatoes and watercress into the food processor.
Add the Earth Balance and the salt and process until smooth and creamy.
You will have to scrape down the sides and pulse it a few times so that all of the ingredients get incorporated. Add pepper to taste.
Serve right away.
Note: Depending on the size of your food processor, you might need to process it in two batches.

1 bunch watercress, chopped into thirds
5 large red potatoes cut into quarters
1/2 cup Earth Balance Buttery Spread
1 teaspoon salt
optional: 2 turns black pepper