Preheat oven to 400 degrees F.
Cut a small sliver off the bottom of each acorn squash so they will sit upright in the baking dish.
Cut the top off of the acorn squash about 1 inch below the stem. Retain the acorn "lid" for later in the process.
Scoop out seeds and stringy flesh and discard.
Place 1 teaspoon of butter in the bottom of each acorn squash.
Place squash on a baking sheet covered with aluminum foil.
Cook farro according to package directions. Drain and set aside when finished.
While farro is cooking, heat 1 Tablespoon of olive oil in a large pan over medium heat.
Add onions, mushrooms and oregano to pan and saute until tender, about 5 minutes.
Add farro and white beans to pan with onion and mushroom mixture and stir to combine.
Season with salt and pepper.
Spoon stuffing into the cavity of each acorn squash. If there is leftover stuffing, and there may be depending upon the size of the squash, serve it on the side.
Place acorn "lid" on top of each acorn squash.
Roast in the oven for 1 hour or until flesh of the squash is tender to the touch of a fork. Check at 45 minutes.
Open lid and sprinkle with feta cheese, if desired.