Peel the sweet potatoes and cut them into 1-inch cubes. Place in a saucepan, sprinkle with 1/2 teaspoon salt and cover with cold water. Bring to a boil and cook until tender when pierced with a knife, 12 to 15 minutes. Drain and return to the pan.

Place the pan back on the heat and allow the water to evaporate, 1/2 to 1 minute. Transfer to a mixing bowl and add the butter, cream and mustard. Stir to combine and once the butter melts, mash all the ingredients together with a potato masher. Stir in the lemon juice, season to taste with salt and pepper and add more lemon juice and/or mustard if desired. Serve immediately.