Make the gremolata. Finely chop the parsley and place it in a bowl with the lemon zest and garlic. Mix in the hazelnuts. Add oil slowly, until it reaches your desired consistency. Stir in the juice of 1/2 a lemon and salt, to taste.
Store gremolata in an airtight container in the refrigerator for up to 3 days.
Preheat your oven to 425°F.
Halve squash lengthwise and scoop out seeds with a large spoon. Using a peeler, remove skin and white flesh.
Rub all over with oil; season with salt and pepper. Roast in a baking dish just large enough to hold halves side by side until beginning to soften (a paring knife should easily slip in only about ¼"), 15–20 minutes.
Meanwhile, bring chilis, maple syrup, butter, and vinegar to a simmer in a small saucepan over medium-high heat, stirring occasionally, until just thick enough to coat spoon, 6–8 minutes. Reduce heat to very low and keep glaze warm.
Transfer squash to a cutting board and let cool slightly. Using a sharp knife, score rounded sides of squash halves crosswise, going as deep as possible but without cutting all the way through.
Return squash to baking dish, scored sides up; season with salt and pepper.
Roast squash again, basting with glaze every 10 minutes, until tender and glaze forms a rich brown coating, 45–60 minutes. Use a pastry brush to lift off any glaze in the dish that is browning too much.
Serve topped with gremolata.
