Cook bacon in a large skillet over medium-high heat until crisp. Drain, reserving 2 tablespoons grease. Crumble bacon, and set aside.
Melt butter with reserved bacon grease in same skillet over medium heat. Add Brussels sprouts, hazelnuts, and green onions. Cook over medium heat, stirring occasionally, 7 to 10 minutes or until sprouts are slightly wilted and tender.
Season with kosher salt and pepper. Toss with crumbled bacon and mint just before serving.