In a large pot melt 2 tablespoons butter. Stir in brown sugar and water. Bring to a boil, then lower and simmer while stirring until reduced and thickened, about 10 minutes.
Peel potatoes and cut lengthwise, then again making long quarters. Cut these into 1-inch pieces ending up with pie slice-shaped wedges. Place in a separate large pot of boiling water and cook for about 5 minutes. Potatoes should give a bit when poked with a fork.
Drain the potatoes and arrange in a single layer in a buttered 13 by 9-inch baking dish. Pour the syrup over the potatoes; making sure it covers the potatoes. Place lemon slices on top and bake for 35 minutes, spooning syrup over potatoes twice during that time. Potatoes are done when they are tender to the fork.
Remove from the oven, remove the lemon slices, and stir in pecans. Arrange the marshmallows evenly on top. Place back in the oven turning it up to 400 degrees F and cook until marshmallows puff up and turn brown, approximately 10 more minutes.