Preheat the oven to 350°F.

Place the beets in a glass baking dish or Dutch oven. Add 1 cup of filtered water
and cover. Roast for 1 hour, until the beets can be easily pierced with a knife or toothpick. Let them cool completely.

Meanwhile, spread the walnuts on an ungreased baking sheet, and toast
in the oven for 5 minutes, or until fragrant. Let cool, then roughly chop and set aside.

Use your hands to remove the skins from the beets, then cut off the tops and bottoms. Slice the beets into thin rounds, about ⅛-inch thick, and set aside.

Peel 4 of the oranges, and cut into rounds: Cut off the tops and bottoms. Stand one orange on a cut end and, using a sharp knife, pare away the rind and white pith in strips from top to bottom, following the contour of the fruit. Lay the orange on its side and cut into ¼-inch-thick rounds, yielding 6 to 8 slices. Repeat with the remaining trimmed oranges.

Grate the zest from the remaining2 oranges into a small bowl. Cut the oranges in half and squeeze the juice into the same bowl. Whisk in the olive oil, thyme, salt, and pepper to make the dressing.

Place the arugula in a bowl and toss with ¼ cup of the dressing. Arrange the dressed greens on a platter, then lay the beets and oranges on top in an alternating pattern. Sprinkle with the toasted walnuts and drizzle with additional dressing.