How to Make It
Preheat the oven to 400°. In a bowl, toss the squash with the garlic, olive oil, rosemary and jalapeño and season with salt and pepper. Spread the squash mixture on a large baking sheet, cover with foil and roast for about 45 minutes, until tender. Uncover and roast until lightly browned in spots, about 45 minutes longer.
Discard the rosemary and mince the jalapeño. Transfer the squash and jalapeño to a bowl. Add the butter, season with salt and pepper and mash.