Special equipment: four 32-ounce wide-mouthed glass jars with lids
Preheat the oven to 350 degrees F. Spread the walnuts out on a rimmed baking sheet, and bake until golden and toasted, 8 to 10 minutes. Let cool, and roughly chop.
Meanwhile, make the barley: Bring a small saucepan of water to a boil. Add the barley, and simmer until plumped and tender, 20 to 25 minutes. Strain and rinse under cold water. (The barley can be made and refrigerated up to 2 days ahead.)
Prepare the grapefruit: Remove the peel and pith from the grapefruits with a sharp knife. Cut the grapefruits into pieces, and remove the seeds.
Make the vinaigrette: Whisk the vinegar, shallots, mustard, honey, 1/2 teaspoon salt and a few grinds of pepper in a medium bowl. Slowly drizzle in the oil, whisking constantly, until blended into a vinaigrette.
Assemble the salads: Using four 32-ounce wide-mouthed glass jars with lids, divide the beets evenly among the jars, then add the following in separate layers: the spinach, Brussels sprouts, barley and grapefruit. Sprinkle the toasted walnuts and pomegranate seeds on top. Pack the vinaigrette in a separate container (about 3 tablespoons per salad).
Just before serving, transfer the jarred ingredients to a large bowl, toss with the vinaigrette and season with salt and pepper.
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