Chamomile-Improves flavor of onion 1
Summer Savory-Improves flavor of onion1
Nightshades (tomatoes, peppers, potatoes, etc)1
Brassicas (cabbage, broccoli, kohlrabi, etc)1
Lettuce is a great planting companion for Onion
Protects lettuce against slugs.
These plants will not tend to grow well with Onion - they could stunt or inhibit growth.