Cut the rolls in half separating the top and bottom. Arrange the bottom half in a single layer in a 20 cm x 30 cm (8-inch x 12-inch) baking pan. Preheat the oven to 190 °C / 380 °F.
Add butter and flour to a saucepan. Place over medium low heat. When the butter melts, stir and cook for about 3 - 4 minutes over low heat. The mixture shouldn't get brown. While continuously stirring with a whisk, pour in 1/3 of the milk to get a smooth paste. Then add the rest of the milk and whisk to get a smooth, silky sauce. Cook for 5 minutes over low heat, then stir in the mustard and prepared horseradish. Season to taste with salt and pepper, and remove from the heat.
make the sliders
Spread the bechamel sauce over the rolls in the baking pan. Place the honey ham on top of the bechamel, then place the cheese on top of the ham. Cover with the other half of the rolls.
In a small bowl, stir to combine melted butter, poppy seeds, sesame seeds, and whole grain mustard. Brush the rolls with the mixture. Cover with aluminum foil, making sure that the foil doesn't touch the rolls. Place in the preheated oven on the middle rack. Bake for 15 minutes at 190 °C / 380 °F. Uncover, increase the heat to 210 °C / 410 °F and continue to bake for another 4 - 6 minutes or until the top of the rolls is golden-brown.
Take the baked Ham and Cheese Sliders with Horseradish from the oven and set aside for 10 minutes, then serve.