Mature onions do have a relatively high natural sugar content, but 1 Tbsp would only have 1g carb .. so I doubt that's the problem. Could it be a taste association?? For example, you used to enjoy onions on a hamburger or sandwich WITH bun .. so the taste of onions makes you crave the bread/bun?? Not sure about the cigs, unless once again there was an association of a habit that included onions at the same time as smoking?

Green onions (scallions) or chives are much lower in sugar & carbs than mature onions, you might try those and see if you have a similar reaction. If you do, then it's definitely a taste association, and best to avoid it.

Doreen