Preheat the oven to 350℉. Without separating the rolls, cut dozen of rolls (they're all stuck together) in half horizontally. Place the bottom half of the rolls in a greased 9x13-inch pan. If you don't have enough room for all 24 in the pan, you'll have to modify with an extra pan.
Layer on top of the rolls: ham, cheese, pickle and another piece of cheese. You'll need to fold up the ham to fit it on top of the rolls. Place the top half of the rolls on top.
In a medium bowl, melt the butter in the microwave. Stir in onion and mustard. Drizzle over the rolls. Cover the rolls with foil and bake for 10 minutes. Uncover and bake for 10 more minutes.
Notes
Make ahead: You can prep these sliders ahead of time and just pop them into the oven when ready to serve.