I mixed the ground chuck with kosher salt, black pepper, garlic powder, and chopped onion until just combined. Then I pressed it into a greased 11×13 pan as an even layer to mimic a giant patty.
Next, I baked the beef slab at 350 °F (about 175 °C) for 25 minutes, until fully cooked (internal temp ~165 °F). I allowed it to rest a few minutes, then trimmed edges and cut into 12 square mini patties.
I sliced the Hawaiian rolls in half (keeping them attached) and spread Big Mac sauce on the bottoms. Then I layered beef, cheese, lettuce, pickles, and more sauce, finished with the top bun. Finally I brushed melted butter mixed with sesame seeds on top and returned the sliders to the oven for 5 more minutes.