Preheat oven to 350°F (175°C). Grease a baking dish with butter.
In a small bowl, mix the mayonnaise and Dijon mustard together. Spread this mixture on the bottom halves of the slider buns.
Layer each bun with turkey breast and Swiss cheese. Spread raspberry preserves on the top halves of the slider buns and place them on top of the assembled sliders.
In a medium bowl, whisk together the milk, eggs, salt, and pepper. Pour this mixture evenly over the sliders.
Brush the tops of the sliders with melted butter. Cover the baking dish with aluminum foil.
Bake in the preheated oven for 20 minutes. Remove the foil and bake for an additional 10 minutes, or until the tops are golden brown.
Remove from the oven and let cool slightly. Dust with powdered sugar before serving.
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