How to Make It
Poke cloves into cut side of onion half. Combine cloved onion half, 1 1/2 quarts water, farro, and bay leaf in a large saucepan; bring to a boil over high. Reduce heat to medium; cook, uncovered, until farro is chewy-tender, about 20 minutes. Drain and rinse with cold water. Discard onion and bay leaf.
While farro is cooking, grate 1 orange to equal 1/2 teaspoon zest, and set aside. Peel oranges with a knife, removing all white pith. Section oranges over a bowl, reserving juice (about 1/3 cup). Squeeze membranes to extract juice; discard membranes.
Place raisins in a small microwavable bowl; pour orange juice over raisins. Microwave at HIGH 1 minute. Place raisins and any unabsorbed juice in a large bowl; let cool slightly.
Combine oil, vinegar, shallot, mustard, salt, pepper, and reserved orange zest in a small jar. Seal and shake to emulsify. Add farro, brussels sprouts, and orange segments to bowl with raisins. Drizzle with dressing; toss gently to coat. Divide salad evenly among 4 shallow bowls. Top evenly with walnuts and cheese slices.