Preheat the oven to 375 degrees F. Line a baking sheet with parchment paper.
Put the squash and red onion on the prepared baking sheet. Drizzle with 1 tablespoon of the oil and salt and pepper to taste. Toss with your hands to coat well and spread out on the baking sheet so they don't steam. Roast, tossing halfway through, until the squash is fork-tender, 30 to 40 minutes. Let cool to room temperature.
Put the raisins in a shallow bowl and pour the vinegar over them. Soak, tossing them every now and then, for at least 15 minutes. Drain the vinegar into another small bowl, pressing down on the raisins to squeeze as much vinegar out as possible. Add mustard, cayenne, and salt and pepper to taste to the vinegar. Drizzle in the remaining 3 tablespoons oil. Whisk until thick and emulsified. Set aside.
To assemble the salad, in a large bowl, add the farro, chicken, cheese, walnuts, raisins, squash and red onions, kale, dressing, and salt and pepper to taste. Toss together until evenly coated with dressing. This is one of those salads that gets better as it sits.