The best replacement for onion is shallot.

The best alternatives to onion are rated as following:
★★★★★  shallot
★★★★  garlic
★★★★  scallion
★★★  onion powder
★★  yellow onion
★  white onion
★  chives
★  garlic first
★  leek
★  red onion
★  green onion
★  tor
  minced onion
  potato
  cooked onion
  brown onion
  sweet onion
  onion
  purple onion
  wild garlic
  spring onion
  artichoke
  fresh onion
  yellow
  tapeworm
  raw onion
  leeks
  shallot taste
  scallions

Your alternative for onion is not listed? Suggest!
Speaking about onion, one probably shouldn't forget about onion powder.

Other search terms related to onion and their ratings are:
★★★★★ onion powder
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Google Trends

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onion
shallot
garlic