Cook farro: In a medium saucepan, bring farro, 1 qt. water, the salt, onion, and bay leaf to a boil over high heat. Cover, reduce heat, and simmer until farro is tender but still chewy, 18 to 25 minutes. Drain farro, discarding bay leaf and onion, and spread on a rimmed baking sheet until cool enough to handle.
Meanwhile, preheat oven to 400°. Roast squash: On a large rimmed baking sheet, combine oil, ras el hanout, and salt. Turn squash in mixture to coat evenly, then arrange flat. Bake, turning once, until tender when pierced, 25 to 35 minutes. Let stand until cool enough to handle.
Prepare dressing and greens: In a large bowl, whisk vinegar and honey to combine. Slowly add oil while whisking. Season to taste with salt and pepper.
Pour one-third of dressing into a second large bowl and set aside. Add farro and mustard greens to first bowl and toss gently to coat. Season to taste with more salt and pepper, then arrange on a large platter. Cut squash pieces in half crosswise (don't peel the tasty edible skin) and gently toss with reserved dressing.
Arrange squash over greens. Scatter cheese on top and gently toss ingredients to mix loosely.
*Find farro at well-stocked grocery stores and natural-foods stores and at bobsredmill.com. Find ras el hanout, a traditional Moroccan blend of 20 or so spices, at well-stocked grocery stores, or substitute equal parts cinnamon, ground allspice, and nutmeg to make 1 tsp. Find ricotta salata, a salty, aged ricotta, at well-stocked grocery stores.