Heat oven to 425°F (218ºC) with rack set in lowest position.
On a rimmed baking sheet, drizzle squash and garlic with oil; toss to coat and season with salt and pepper. Arrange squash in a single layer, cover tightly with foil, and roast until squash is just starting to soften, 12–15 minutes.
Remove foil, increase oven temperature to 450ºC (232ºC) and continue to roast squash until side touching the pan is golden brown, about 10–12 minutes. Remove sheet from oven and using a thin spatula, flip squash. Return to oven and continue to roast squash until side touching the sheet is golden brown, about 10–12 minutes more.
Meanwhile, melt butter and pepitas in a large skillet over medium-low heat. Cook, stirring frequently, until butter and pepitas are golden brown and fragrant, 4–5 minutes. Immediately remove from heat and stir in maple syrup and coriander (mixture will bubble). Let cool 1 minute. Meanwhile, remove skins from roasted garlic and smash into a paste. Stir mashed garlic and vinegar into brown butter mixture; season with salt and pepper to taste.
Transfer squash to a serving platter and drizzle with vinaigrette and pepitas; sprinkle parsley and sea salt over top.