Preheat the oven to 400 degrees Fahrenheit.
Place the cubed butternut squash on one large baking sheet, and the sliced red onions on another. Drizzle both with olive oil & season with a generous sprinkle of kosher salt and freshly cracked black pepper. Use your hands to toss the veggies to evenly coat with oil and seasonings.
When the oven is done preheating, add the tray of butternut squash and the tray of onions to the oven. Cook for about 45 minutes, stirring occasionally for even browning. The squash is done when it is fork-tender and lightly browned, and the onions are done when they are nicely caramelized and tender.
Meanwhile, bring a large pot of salted water to a boil. Add the farro & cook according to package instructions (either 12 minutes for quick cooking, or about 1 hour for regular farro).
When the farro is tender, drain & place in a large serving bowl.
When the squash and onions are done cooking, transfer to the large bowl with the farro. Toss to combine. Add the sunflower seeds, pepitas, cranberries, and cruciferous veggies, and toss again to combine.
In a small skillet, melt the butter over medium heat. Add the sage leaves and cook, stirring continuously, until the melted butter is golden brown (but not burned). Turn off the heat & add the juice of 1 lemon. Stir, then pour over the farro salad. Toss to combine and serve right away, topped with freshly shaved Parmesan cheese if you want!