In a medium saucepan, combine the coconut milk, lemongrass, ginger, garlic, chiles, lime zest, coriander seeds, and salt. Bring to a boil, then reduce the heat to maintain a simmer. Cook until reduced by half, then strain through a fine-mesh sieve. Discard the solids.
Arrange the squash on a serving dish, overlapping the pieces slightly. Squirt lime juice all over the squash, then spoon the sauce on top. Garnish with the cilantro and scallions.
Preheat the oven to 350°F. Cut the squash in half lengthwise, then cut crosswise into ½-inch-thick half-moons. Transfer to a half sheet pan and toss with the oil, salt, garlic, and chile flakes. Spread in a single layer. Roast until tender and browned, about 30 minutes.