Found this recipe on the web. Honestly, I've never had a White Castle slider but the photos looked so amazing I just had to keep this recipe and share it. They can be prepared ahead of time and reheated.....see 'note'.
Evenly spread the onions on the bottom of a 9x13 inch baking dish.
Gently press the ground beef on top of the onion flakes.
Press evenly making one giant patty.
Sprinkle with seasoned salt.
Bake in a 400°F oven for 20 minutes.
Using a paper towel, pat the excess grease from the top of the patty.
Top with cheddar cheese, return to oven for 2 more minutes.
Remove from oven, let set 5 minutes then cut into 24 patties.
Slice open and separate each party roll.
Place bottom of rolls on a large platter.
Using a spatula lift patty with onion layer onto each bun bottom.
Top each slider with a pickle slice then the bun lid.
Makes 24 sliders.
Note- To make ahead -- store upside-down in an airtight container in the refrigerator. This will prevent the bottom bun from getting soggy from the onion layer. To reheat, place a few on a microwave safe plate and heat in the microwave for 20 seconds then 5 second increments after that until warm through.