Boil the sweet potatoes until very tender. Drain well, thoroughly mash, and then whip in the butter, milk, salt and pepper. Preheat the oven to 350 degrees F. Combine the remaining ingredients in a bowl and then spoon into 2 small baking dishes, each with a 1 to 1 1/2 cup capacity. Top the lentil mixture with the sweet potatoes. Bake 35 to 40 minutes, or until golden and bubbly.