Over medium-low heat, sweat down the leek whites, garlic and green onions whites in 1 Tablespoon of olive oil. Add a pinch of salt and cook until soft and translucent, but not colored (about 6 - 8 minutes). Slice the potato, then add to the pan and top with enough water to cover the vegetables (approx 2 cups/ 500 ml). Simmer for 30 minutes. Carefully transfer the pan contents to the Vitamix container. Start the blender on its lowest speed and increase to its highest speed, blending for 1 minute. Reduce the speed to Variable 2. Remove the lid plug and add the green onion greens, mint and watercress through the lid plug opening. Increase to its highest speed and blend for 30 seconds. Season with salt, pepper and add fresh lemon juice. Serve immediately, or if serving chilled, cool down rapidly to preserve the lovely bright green color.