Preheat the oven to 400°F. Lightly grease a large rimmed baking sheet or shallow roasting pan.
Pat the chicken thighs completely dry with paper towels. Arrange them skin-side up on the prepared baking sheet and drizzle with olive oil.
In a small bowl, combine the kosher salt, garlic powder, ground mustard, onion powder, dried sage, paprika, and black pepper. Sprinkle the seasoning mixture evenly over the chicken thighs, gently rubbing it into the skin.
In a large bowl, combine the cubed butternut squash and sliced red onion. Drizzle with olive oil, then sprinkle with kosher salt and black pepper. Toss until evenly coated.
Spread the seasoned vegetables around the chicken thighs in a single layer, leaving enough room for the vegetables to roast rather than steam. Use two baking sheets if necessary.