Preheat the oven to 350 degrees. Melt the butter in a large pot. Stir in the brown sugar and 1/2 cup water. Bring to a boil, reduce heat to medium and simmer, stirring, until reduced and thickened. Approximately 10 minutes.
Bring a large pot of water to a boil. Meanwhile, peel the potatoes and cut them lengthwise, then lengthwise again, making long quarters. Cut the quarters into 1" pieces or keep long quarters, making sure the pieces aren't too thick.
Boil the potatoes until they give a bit when poked with a fork, about 5 minutes.
Drain the potatoes and place in a greased 2-1/2 quart baking dish.
Pour the syrup over the potatoes, making sure to cover all the potatoes.
Place the lemon slices on top of the potatoes.
Bake 30-35 minutes or until the potatoes are fork-tender. Spoon the syrup over the potatoes 2-3 times during the baking time.
Remove from the oven, discard the lemon slices and stir in the pecans.
Turn the oven up to 400 degrees, top with marshmallows and place back in the oven. Cook until marshmallows puff and turn brown, approximately 10 minutes.
Find it online: https://livelaughrowe.com/best-sweet-potato-casserole-recipe/