Zest lemon and add zest to large saucepan. Juice zested lemon and add juice to the same saucepan along with the lemon peel.
Add the apricots, sugar, and water then bring the mixture to a simmer over medium heat. Cook until the fruit is soft and the sugar is dissolved, about 20 minutes. Skim foam if necessary. Remove from the heat, cover and hold in the refrigerator for at least 6 hours or overnight.
While the apricots are cooking, smash the brown and yellow mustard seeds with a mortar and pestle or coffee grinder until most of the seeds are broken and slightly ground (not to a powder). Put the smashed seeds and ground mustard in a small bowl, pour in the apple cider vinegar, and set aside, covered, on the countertop, at least 6 hours or overnight.
The next day
Sterilize clean 4 to 8 oz canning jars and keep them hot. Put a small plate in the freezer. Remove the lemon peels from the fruit mixture, pressing out the remaining juice and pulp. Return the saucepan of fruit to medium heat on the stovetop and cook down until thickened and amber in color, about 30 minutes. Stir in the vinegar-mustard seed mixture until combined. Scoop out about a cup of the apricot mustard and purée in a blender, on high speed until smooth, then return to pot OR puree the mixture lightly with an immersion blender. Cook until thick and set, about 15 to 30 minutes. Skim foam as necessary.
To test the set, remove the plate from the freezer and add a small spoon of mustard to the plate. Push the mustard with your fingertip. It should wrinkle, indicating it has set. If the mustard is loose, return the mixture to the heat and cook for another 10 minutes, checking the set until the desired consistency is reached.
Ladle hot mustard into hot jars leaving 1/4 inch headspace. Remove air bubbles. Wipe rim. Center lid on jar. Apply band until fit is fingertip tight.
Process jars in a boiling water canner for 10 minutes, adjusting for altitude if over 1000 ft. Remove jars and cool. Check lids for seal after 24 hours. Lid should not flex up and down when center is pressed.
Notes
Makes 4-5 cups. If you have never canned before, you should familiarize yourself with the techniques using a good canning guide like the Ball Home Canning Book.
I am not a health professional and nutrition data is calculated programatically. Accuracy may vary with product selection, calculator accuracy, etc. Consult a professional for the best information.