Spread sauerkraut into bottom of a 9-inch square glass baking dish.
Stir reserved liquid from sauerkraut, brown mustard, brown sugar, celery seed, and black pepper together in a bowl; drizzle over sauerkraut. Arrange sausage slices in a layer over the sauerkraut and sauce. Loosely pile frozen french fries atop the sausage.
Bake in preheated oven until the french fries are lightly browned, 20 to 30 minutes.