Preheat the oven to 450F or 425F convection. Line two rimmed baking sheets with parchment paper.
Cook the farro and lentils if you don’t have them prepped. Add farro to a medium pot and add water to cover by 2-inches. Bring to a boil, then simmer for 15-20 minutes or until cooked through. For the lentils, add them to a medium pot and add enough water to cover by 2-inches. Bring to a boil, then simmer very gently for 10-12 minutes, or until lentils are tender but not mushy. Drain each and set aside.
Add beets to one baking sheet and fennel + onion (if using) to the other. Drizzle each with 1 tablespoon olive oil and ¼ teaspoon salt and toss to coat. Place on the middle and bottom racks and bake for 15 minutes, or until fennel is browned and charred in spots and beets are tender, switching the baking sheets halfway through.
Add all tahini dressing ingredients to a medium bowl and whisk thoroughly to combine (it will turn creamy I promise). If too thin, whisk in a bit more tahini. If too thick, whisk in some water. Taste and add more salt, lemon, or maple, if needed. Dressing will thicken as it sits and cools in the fridge. Stir some water in to thin it out.
Add farro, lentils, and roasted veggies to a bowl. Drizzle with the tahini dressing. Sprinkle with a pinch of salt and top with toasted nuts, parsley, and a squeeze of lemon.