it depends on how many stock cubes you use and how big a portion your lo is going to eat. Is the casserole just for your lo or is it for everyone? If it is for everyone and your lo is only having a small amount then as a one off it would be ok.
Whilst you need to keep the amount of salt in a baby's diet to an absolute minimum, I think in the long term it is going to do no harm. A chunk of cheese probably contains more salt. I think we have been sold the idea that babies need completely different food by the baby food manufacturers, who are trying to make a profit. I think people who cook the same meal for everyone including the baby are starting the child off in absolutely the right way - and it is a good plan to look at the family's diet as a whole, as the ultimate goal is for everyone to be eating well, not just for the first year or so of their lives.
I use low salt kallo or Marigold boullion anyway, don't add salt to veggies etc, and have never got worked up about the salt thing - if they aren't eating loads of packaged foods, it won't be a problem.
Far better to be eating a varied, tasty diet with the rest of the family than getting into the thing of not eating what others do. And making the rest of the family eat more healthily if necessary.