Add the sliced onions to a medium heavy-bottomed saucepan or dutch oven, along with the chopped garlic (if using).
Add enough honey to cover the onions. They do not need to be entirely submerged, but the honey level should be as high as the onions.
Turn the burner on low and allow the onions and garlic to soften, stirring occasionally.
Cook on low heat until the onions are soft and slightly mushy, about 1-2 hours. Stir occasionally to encourage even heating.
If your onions are still not very soft, you can increase the heat slightly to encourage them to release their juices. The goal is to keep the heat low to preserve the enzymes and benefits of honey.
Once the honey has been infused, turn off the heat and allow it to cool to room temperature.
Place the honey onion cough syrup in a glass jar, and top it with a lid once the honey is no longer warm.
Store in the refrigerator for up to 1 week with the onions and garlic, or strain off the onions and garlic if you would like to store for 2-6 months. Discard if there are any signs of spoilage such as mold or an off smell.
Take ½ - 1 teaspoon every 1-2 hours at the first signs of a cold. If you’re treating a cold that has already started, take 1 teaspoon 3-4 times per day.