[{"ProjectID":"555","Content":{"CustInfo":[{"Id":"123","CustomerComment":{"Id":"1","notes":"This is a note."}},{"CustomerComment":{"Id":"2","notes":"This is a note 2."},"Id":"124"}]}}]

db.collection.aggregate([{$match:{"ProjectID":"555","Content.CustInfo.Id":"123"}},{$unwind:"$Content.CustInfo"},{$match:{"Content.CustInfo.Id":{$eq:"123"}}},{"$replaceWith":"$Content.CustInfo.CustomerComment"}])