Coarse Ground Mustard
Peel and halve the onions and garlic. Peel the apple, cut into quarters, and remove the seeds. Add the vinegar to a saucepan with the apple, onion, garlic, cloves, peppercorns, and bay leaves. Bring the mixture to a boil. Reduce the heat, cover the pan, and simmer for 15 minutes. Pass the vinegar mixture through a fine strainer into a clean pot, then add the mustard seeds and bring to a boil. Add the mustard powder to a bowl. Crush the horseradish seeds in a mortar, then add to the bowl. Gradually incorporate the vinegar mixture to the bowl, whisking to combine completely. Set aside and let the mixture infuse for 5 minutes. Fill decorative, sterilized jars to the brim with the mustard and seal tightly.
Label the jars accordingly. The mustard will last for up to 6 months sealed in the jars.