Preheat oven to 400°. Line a rimmed baking sheet with parchment paper.
Place 8 slices of bread on lined baking sheet and butter one side. Flip buttered side over so that it is facing down.
In a large bowl, combine all remaining ingredients.
Evenly divide Ruben mixture between the 8 slices of bread.
Spread the mixture so that it evenly covers each slice to the edge.
Place open-faced sandwiches in the preheated oven and bake for 20 minutes or until the cheese is melted and golden.