Cut the onion into thin slices. Add to a glass, heat-proof jar. Set aside.
In a small saucepan placed over medium-low heat, combine the vinegar, water, and sugar. Stir to dissolve the sugar. Heat for 3-4 minutes. Pour the liquid over the onion slices. Let cool and marinate for at least 30 minutes. Refrigerate for up to one week.
Preheat the oven to 375 degrees.
In a small bowl, stir together the mayonnaise and horseradish. Season to taste with salt and pepper. Set aside.
Place the bottom half of the slider rolls in the baking dish and spread them with half of the horseradish mayo.
Top with the cheddar cheese followed by the roast beef, pickled onions, and Provolone cheese.
Spread the remaining mayo mixture on the inside of the top of the slider rolls. Place the rolls on top of the sliders.
Melt the butter in a small saucepan. Stir in the Everything bagel seasoning. Brush the butter mixture over the top of the rolls.
Cover the baking dish with foil and bake for 30 minutes or until the cheese is melted.
The pickled onions and horseradish mayo can be made ahead of time. Both will keep well for several days in the refrigerator.
Use thinly sliced beef to create a tender, easy-to-eat sandwich.
To prep ahead of time, assemble the sliders but stop before brushing the rolls with the butter mixture. Cover tightly and refrigerate for 1-2 days. When ready to bake, brush the rolls with butter, cover the dish with foil, and bake according to the recipe directions.
To reheat easily and quickly, wrap one or two sliders in a paper towel and microwave for 40-50 seconds until warmed through. The amount of time will vary depending on your own microwave.