Preheat the oven to 400°F. Toss the squash cubes with 2 tablespoons olive oil, salt, and pepper on a baking sheet. Roast for 25 to 30 minutes until the edges turn golden and the squash is tender. Set aside to cool.
While the squash roasts, bring the water to a boil in a medium pot. Add the farro and reduce to a simmer. Cook for 20 to 25 minutes until the grains are tender but still chewy. Drain any excess water and let the farro cool to room temperature.
Place the cooled farro in a large bowl. Add the roasted squash, sliced red onion, parsley, and mint.
Whisk the remaining 1 tablespoon olive oil with the lemon juice. Pour over the salad and toss gently to coat everything evenly.
Sprinkle the feta cheese over the top just before serving.