1. Preheat oven to 425°. Arrange beets in single layer in a 13- x 9- x 2-inch baking dish. Cover dish tightly with foil and roast beets until tender (1 to 1 1/4 hours). Once cool enough to handle, remove skin and cut each into 6 to 8 wedges.
2. While beets roast, cover farro with cold water in a bowl and soak 30 minutes; drain, discarding water. Bring farro, 4 cups water, and 1/4 teaspoon salt to a boil in a medium (2- to 3-quart) saucepan; reduce heat and simmer, uncovered, until just tender (15-20 minutes). Drain.
3. Whisk together vinegar, honey, and pepper until combined; add oil in a stream, whisking to combine.
4. Transfer farro to a large bowl. Stir half of dressing into hot farro; add beets, arugula, almonds, and remaining vinaigrette, and stir gently to combine.