Preheat the oven to 425F.
Trim both ends off each delicata squash. Cut each in half lengthwise, scoop out the seeds, and slice them into 1/4-inch thick half-circles.
Toss the squash with the oil, salt, and black pepper, and spread it out evenly on a large baking tray.
Roast the squash until its golden brown in spots on both sides, about 25 to 30 minutes, flipping once halfway through.
After you flip the squash, spread the hazelnuts out in an even layer on a small baking tray and roast until light golden, about 5 to 10 minutes. Cool, and then coarsely chop the hazelnuts.
Meanwhile, add the butter to a small saucepan over medium heat. Once melted, add the rosemary sprigs and cook until the butter is light brown and smells nutty, about 5 minutes. Remove and discard the rosemary.
To serve, put the squash on a serving platter. Drizzle on the rosemary browned butter and sprinkle on the hazelnuts. Garnish with fresh rosemary if desired.