Assembling sliders ahead of time is a great way to save time later. It is a good make-ahead appetizer, snack, or lunch. I pre-assemble sliders for parties all the time. I suggest assembling the sandwiches and placing them in the refrigerator. When ready to serve, add the topping, cover, & bake. You may need a few more minutes in the oven since the ingredients are cold versus room temperature.