I would like to make bread and butter pickles. I already have the sliced cucumbers, onion, and salt sitting in the fridge! However, my son is allergic to all forms of mustard. Can I simply leave the mustard seed out or is there something else it can be replaced with?
Mustard seeds and other spices are there for flavor only; it's perfectly safe to leave them out. But don't mess with amounts of salt or vinegar given in your recipe--those are important for preventing bacterial growth.
If your recipe also contains bay leaf or grape leaf, those too can be omitted, but you pickles won't be as crisp. Likewise if you omit the calcium chloride or calcium hydroxide.