I found this for you from wholesomebabyfood.com
Most often we are asked about Onions, Mushrooms, Celery, Leeks and Peppers.
These veggies may be sautéed and added to your child's food between 8-10 months old. We do not recommend offering your baby an onion, mushroom, celery or a pepper puree rather we recommend that you add these wonderful veggies to food dishes. Should you decide to offer your baby onions, mushrooms, celery and peppers, please ensure that they are fully cooked, soft, and appropriately sized so as not to pose any choking hazard.
It is most important to cook mushrooms for your baby's food. Mushrooms as we all know are fungi and cooking helps ensure their safety.
These veggies do not pose a high allergy risk however there may be certain digestive issues in a sensitive or delicate tummy. We think that these veggies are best served in soups, casseroles, noodle/rice dishes. Our babies loved sautéed mushrooms with sauteed spinach. This would be for the older baby (10 months +) due mostly to the spinach.