All you need are a few onions, so watch for them on sale, or save up any onion pieces you don’t use as you are cooking (such as the root ends, middles and skins) and keep them in a container in the refrigerator. You will need 3-4 cups of onion pieces.
In a pasta pot of water, bring onion pieces to a boil. Allow to boil for 3 minutes, then cool. Pour into plastic container with a lid, and allow to “steep” for a few days. Strain out the onion pieces and pour the onion water into a spray bottle.
Simply spray onto plants that are having insect issues. If you have plants prone to aphids, you can bury some of the strained onion pieces in the pot, making sure you cover with soil.