One of the 14 ingredients that must be highlighted on EU food labels.
Probably more common in France, not much data for UK but seems rare.
One of the spices that has cross reactivity with mugwort so look for other foods causing problems, as in celery spice mugwort syndrome.
Used in lots of different cuisines across the world eg black mustard seeds in curry, not just mustard as used on hot dogs and sandwiches. Also a key ingredient in:
- mayonnaise (more usually the egg causes problems but could be either or both),
- ready meals and other prepared foods,
- Salad dressings,
- Honey and mustard sauce eg for chicken, gammon,
- Pickles eg gherkins
Sometimes mustard leaf gets used as a vegetable!