Onions can be grown easily either indoors in containers or outdoors. It helps in growth of fruit trees, nightshades (tomatoes, peppers, potatoes), brassicas, carrots and repels slugs, aphids, carrot fly, cabbage worms. They should not be planted along with beans, peas and parsley.You can harvest onions at any stage. The plants you thin from a row can be used as green onions. However, they are ready when about ½ the tops have fallen over and the bulbs’ skins have a papery feel.
9 Tricks for Growing Onions- "it’s always a good idea to grow onions in their own bed away from other plants. That way the onions will get the curing time to dry. But if you plant onions among other plants that need water, you can rot the onions. "