**Preheat Oven:** Preheat your oven to 350°F (175°C). Grease a baking dish or line a baking sheet with parchment paper.
**Prepare Buns:** Slice the slider buns in half horizontally, keeping them connected. Place the bottom half in the prepared baking dish.
**Assemble Sliders:** Spread a thin layer of mayonnaise and Dijon mustard (if using) on the bottom buns. Layer with turkey, cranberry sauce, and cheese slices. Place the top half of the buns over the fillings.
**Make Butter Mixture:** In a small bowl, combine melted butter, garlic powder, parsley, and poppy seeds.
**Brush and Bake:** Brush the butter mixture generously over the top of the buns. Cover the dish with foil and bake for 10 minutes. Remove the foil and bake for an additional 5-7 minutes, or until the tops are golden and the cheese is melted.
**Serve:** Remove from the oven, let cool slightly, and slice into individual sliders. Serve warm.