In a large bowl, mix together the ground beef, beefy onion soup mix and beef broth until well combined well.
On a piece of parchment paper or wax paper, roll the hamburger meat out to about ¼" thickness. Just keep rolling and make the meat into a rectangle shape. If you need to eyeball it, use the slider buns to size 4 across, 3 down - that's how big your beef patty should be when it's done. Remember that White Castle patties are quite thin. Now using a knife, cut into the meat where the slider buns will go making 12 distinct patties but keep them together as one large rectangle still, just make the cuts not fully separating them. Next, using anything that can make a decent sized hole (we use the handle of a wooden spoon) make 5 holes in each patty.
Place the meat in freezer for 25 minutes. It will not freeze but it should be a little bit easier to handle when it comes out.
In a large baking pan, pour 1 cup of beef broth and 3 tablespoons of minced onions and stir gently spreading the onions out evenly. Place the giant meat rectangle into the liquid. The liquid steams through the burgers and cooks them perfectly. Put in the 450 degree F. oven for 10 minutes.
Remove from oven after 10 minutes and drain all liquid from pan. (its ok for residual liquid to be there. Turn the meat out onto another piece of parchment paper.
On the baking pan, place the 12 BOTTOMS of the slider buns. Now gently turn the parchment paper back over placing the hamburger back on the buns. Using the cut marks you made as a guide, separate the burgers. Top with cheese slices, pickles and put the TOPS of the slider buns on.
Return to oven for 3-5 minutes, just long enough to melt the cheese. Serve immediately.