Mustard plants can grace your table in a couple of ways. Besides mustard greens, you can also harvest the seeds from the plant for use as a delicious spice, as well as for planting next year's crop. Mustard seeds can also be used to make any recipe for the popular condiment called "mustard" that you would like. They will first need to be dried, but after you have done that, you can store them in your pantry and use them as needed.
Wait until your mustard plants begin to turn yellow. This will happen late in the mustard's life cycle, after it has grown quite large. If you like to eat mustard greens, you will probably have already harvested and eaten some of the leaves prior to their turning yellow. Ideally, the seed pods should have time to mature, but not so much time that they burst open and you lose seeds.
Collect the seed pods and carefully open them. You can do this with your hands. Empty the seeds from the pod into your bowl. Repeat steps 1 and 2 for as many seed pods as you would like to gather.
Empty the bowl of fresh mustard seeds onto a sheet pan, or multiple sheet pans if you have a lot of seeds. Spread them out evenly, so that there is only one layer on the pan. It is OK if some of them touch, but ideally you should try not to crowd the seeds. Reserve a few for planting next year by putting them in a small zip-top bag and marking it accordingly so you know what it is when you are planting next year's garden.
Turn your oven to the lowest setting it will go, which is usually around 200 degrees Fahrenheit. Let it preheat to that temperature.
Turn your oven off when it has finished preheating. Mount your oven thermometer inside, and keep an eye on it. When the thermometer indicates that the temperature has dropped below 150 degrees Fahrenheit, it is safe to place your sheet pan(s) of mustard seeds inside the oven.
Dry the seeds for a few days. You will not need to turn the oven on more than once. If you need to use your oven for cooking other food, remember to remove your sheet pans of mustard seeds first. Continue cooking on your stove top as normal. The residual heat from the stove top will help to speed the drying process.
Store your mustard seeds in an airtight container. Place the container in a cool, dark place, such as the back of a cabinet or pantry.