Preheat oven to 425°

Spread onions evenly on a large cookie sheet or half sheet pan.

Place ground chuck on top of onions.

Spread and press down meat to evenly cover onions. This can be prepared ahead of time. Cover with plastic wrap and store in the refrigerator for up to 24 hours.

Cook sliders in preheated oven for 25 minutes. Meat will shrink up.

Remove sliders from oven and drain excess grease.

Set meat aside and toast buns in the oven for 2 minutes.

As buns toast cover meat with cheese. Remove buns from the oven.

Place sliders in the oven until cheese melts.

Cut meat into 24 squares.

Place meat on buns and serve immediately.