Step 1 In a bowl, combine the mustards, one-fourth teaspoon salt, one-eighth teaspoon pepper and the vinegars.
Step 2 In a measuring cup with a lip, combine the grape seed oil and the water. Whisk the mustard-vinegar mixture until smooth, and then, while adding the oil-water mixture in a thin stream, continue to whisk. Continue whisking and pouring slowly until all the oil has been emulsified into the dressing. Use the dressing as needed. Keep it in a squeeze bottle, or small water bottle fitted with a squirt top, in your refrigerator. (Makes about three-fourths cup.)