Cut both ends off the squash and cut it into 2 sections: the cylindrical top and the bulbous bottom. Peel both sections, then cut the bottom in half lengthwise and scoop out the seeds with a spoon. Cut the flesh from both sections into 1-inch chunks (or cubes if you’re feeling fastidious). You should have about 5 cups.
Put the butter in a large skillet over medium heat. When it melts, add the garlic and ginger and cook, stirring occasionally, until they soften and turn golden, 2 to 3 minutes. Add the curry powder and stir until it becomes fragrant, just 1 minute or so.
Stir in the squash, coconut milk, and some salt and pepper. Turn the heat to high, bring to a boil, then lower the heat to a gentle bubble. Cover and cook, stirring once or twice, until the squash is tender (you’ll be able to pierce it easily with a sharp, thin-bladed knife), 15 to 20 minutes.
Remove the lid and raise the heat to medium-high. Cook, shaking the pan occasionally and stirring the squash just a few times, until the liquid thickens a bit, no more than 5 minutes. Taste and adjust the seasoning, garnish with the cilantro and scallions, and serve with the lime wedges.