In a medium pot, add the farro and enough water to cover by 2 inches. Bring to a boil over high heat, then reduce the heat to medium-low and simmer rapidly for 20-30 minutes, or until tender. Drain and transfer to a large bowl to cool.
In a large bowl, whisk together the tahini, water, lemon, oil, mustard, fish sauce, garlic, and salt until smooth and creamy. The consistency should be creamy, but very drizzle-able. Thin out with more water if it’s too thick (this can vary depending on the thickness/brand of the tahini).
Add the greens (if using kale, massage it with a touch of oil to soften it before adding everything else), farro, fennel, and cucumber. Toss well to coat.
Serve topped with walnuts, parmesan, and black pepper. See "leftovers" section for tips if you prepped this ahead.