The nutritional profile of onions depends on the species and variety. If you're looking for average values, here they are:
89% water
11% solid organic matter
9% carbohydrates
1% protein
Negligible amounts of fat
If you're thinking about including onions in your diet, know that they have a relatively low energy value of about 166 kJ (40 kilocalories) per 100 g (3.5 oz).