Researchers at Clemson University report on garlic as 3% of poultry feed. Strangly enuf, they were trying to mitigate the odor of large, commercial outfits but on the subject of the taste of the eggs: "The researchers have already conducted taste tests and found that people preferred the eggs produced by the garlic-eating hens."
My hens have often had onions in kitchen scraps and have eaten them . . .
It seems to me there would be almost no nutritional value to a chicken. Better to put onion scraps in your compost where they can contribute organic matter.