Partially cook the squash: either roast in the oven or parboil for a few minutes in salted water (this can be the same water you are using to cook the farro (see next step). If roasting the squash, preheat oven to 400° F. Place squash cubes on a rimmed sheet pan and drizzle with 1 tablespoon avocado or olive oil. Roast for 15 minutes.
Cook the farro: Bring 6 cups water to a boil in a medium sized saucepan with lid. Rinse farro under running water, add to pot with 1 teaspoon salt, bring back to boil, lower heat to maintain a gentle simmer and cook until the farro is just tender, "al dente", about 15 minutes. Drain off water and set aside. (Will result in about 3 cups cooked farro.)
Heat the butter and olive oil in a large, deep sauté pan or dutch oven set over medium-high heat. Add the onions and sage and cook for 10 minutes, until the onions are tender.
Add the cooked farro and broth to the onions, and bring to a boil. Add the chestnuts and partially cooked squash, and stir to combine. Reduce heat to low, cover the pan leaving a small air gap, and cook at a slow simmer until most of the broth as been absorbed. Stir in the butter and season with salt and pepper.