Peel your butternuts and cut in half. Scoop out the seeds (and put aside). Make a series of cuts every ¾ inch down your squash but do not cut all the way through. If you put two cutting boards on either side of your squash your cuts will not go through. Be careful not to break the squash. Easy to do if the butternut is not perfectly straight.
When you have made your cuts, put a clove of garlic (or two if they are small) between each cut. Drizzle with olive il and roast at 375 for 50-60 minutes until a knife slips in easily. (Times for smaller squashes will be less. Ovens vary as well. Check your squash 10-15 mins before the expected end of cooking time.)
While the squash is roasting, prepare your farro on the stovetop according to package directions. You will need 3-4 cups of finished farro. When cooked til firm but not crunchy, drain and set aside.
In a large heated skillet on medium heat, add 2 tablespoons of olive oil and sauté the onion until translucent. Add minced garlic. Sauté for 1-2 minutes. Add a quarter cup of dry white wine or broth and the dried cranberries to deglaze the pan and sauté for 2-3 minutes to cook off the alcohol. Add the cooked farro and combine well. Add the arugula in large handfuls and mix. Add more arugula as the cooking wilts it down. Salt and pepper to taste. Take off the heat and toss in the toasted nuts mixing thoroughly to combine.
When the squash is tender and still hot from the oven, brush with real maple syrup and dot with butter. If your hasselbacked butternut is still intact, place on a platter and spoon heaping helpings of the farro mixture between the slices and around the squash. If your hassleback attempt falls apart, serve each squash slice with a generous amount of farro and enjoy! Serve warm.
Optionally, you can make a drizzle sauce of 2/3 real maple syrup to 1/3 melted salted butter and serve to drizzle over your stuffed squash.