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Preheat oven to 425°
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Spread onions evenly on a large cookie sheet or half sheet pan.
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Place ground chuck on top of onions.
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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.
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Cook sliders in preheated oven for 25 minutes. Meat will shrink up.
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Remove sliders from oven and drain excess grease.
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Set meat aside and toast buns in the oven for 2 minutes.
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As buns toast cover meat with cheese. Remove buns from the oven.
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Place sliders in the oven until cheese melts.
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Cut meat into 24 squares.
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Place meat on buns and serve immediately.