Step 1In a bowl, combine the mustards, one-fourth teaspoon salt, one-eighth teaspoon pepper and the vinegars.
Step 2In 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.)
Step 1Heat oven to 350 degrees. Place sliced beets in one layer in a 13-by-9-inch baking dish. Pour the vegetable stock and the oil evenly on top. Cover the pan and roast until the beets are tender, about 40-45 minutes. Remove the beets from the pan and let them cool for 20 minutes. Reserve the pan juices.
Step 2Mix the red onion, diced tomato and avocado together with 1 tablespoon of the whole-grain mustard dressing. Add the cumin and salt, then stir.
Step 3Place one slice of beet on each plate and add one-half tablespoon of the tomato-avocado mixture, then another beet, then mixture, then beet, creating layers. Spoon one-half tablespoon of the reserved beet juice over each stack. Drizzle additional dressing over the top and around each napoleon.