I eat one whole onion bulb a night with my dinner. To help with the bad breath/aftertaste, I complement it with yogurt (I know, not paleo, but a staple in my diet regardless) and also chow on some parsley.
Today, I decided to do some research into the majestic onion and yielded this:
Blood Sugar-Lowering Effects
The higher the intake of onion, the lower the level of glucose found during oral or intravenous glucose tolerance tests. Experimental and clinical evidence suggests that allyl propyl disulfide is responsible for this effect and lowers blood sugar levels by increasing the amount of free insulin available. Allyl propyl disulfide does this by competing with insulin, which is also a disulphide, to occupy the sites in the liver where insulin is inactivated. This results is an increase in the amount of insulin available to usher glucose into cells causing a lowering of blood sugar.
In addition, onions are a very good source of chromium, the mineral component in glucose tolerance factor, a molecule that helps cells respond appropriately to insulin. Clinical studies of diabetics have shown that chromium can decrease fasting blood glucose levels, improve glucose tolerance, lower insulin levels, and decrease total cholesterol and triglyceride levels, while increasing good HDL-cholesterol levels. Marginal chromium deficiency is common in the United States, not surprising since chromium levels are depleted by the consumption refined sugars and white flour products as well as the lack of exercise. One cup of raw onion contains over 20% of the Daily Value for this important trace mineral.