Cut the bulbous bottom of the squash off from the long top. Split the bottom in half through the root end and scoop out the seeds with a spoon. Cut the long part in half lengthwise. Peel or cut off the rind, then cut the flesh into 1-inch pieces.
Add the squash to a 4-quart pot, add enough water to cover, bring to a boil, and cook, covered, over high heat, about 18 minutes. Add 1 tablespoon salt and cover. Lower the heat to medium and cook until the squash is tender when pierced with a fork, 10 to 12 minutes more. (There should be no resistance, but the squash should not fall apart. Be attentive: It can overcook very quickly and absorb too much water, making your mash soupy.)
Meanwhile, toast the coriander in a small dry skillet over medium-high heat, stirring often, until aromatic, 1 to 1 1/2 minutes. Transfer to a small bowl.
Strain the squash in a colander. Return it to the pot, add the butter and mash with a potato masher until smooth. Add the zest, 1 tablespoon of the maple syrup, 1 1/2 teaspoons of the toasted coriander and salt to taste and stir to combine. When serving, drizzle the remaining 1 tablespoon syrup over the top and dust with the remaining 1/2 teaspoon coriander.
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