Makes 4 cups.
1 cup light or dark mustard seed
6 Tbsp dry mustard powder, lightly packed
1-1/3 cups water
1-1/2 cup rice vinegar
6 Tbsp mild-flavored honey
4 tsp salt, or less to taste
2 Tbsp chopped fresh tarragon leaves
Combine mustard seed, mustard powder and water in a food processor or blender, and process to a coarse puree. Let the mixture stand uncovered, at room temperature, for at least one hour and up to four hours. Stir occasionally. Combine mustard mixture with the vinegar, honey, salt and tarragon. Process in food processor or blender to the texture that you like, coarse or creamy. Store in clean, dry jars, tightly capped in a cupboard, where it will mellow gradually. The mustard will be ready to use in a few days.