Trim stem off squash, and cut squash crosswise at base of neck to separate bulb and neck. Trim squash on all sides to create a (2 ¾-inch by 5-inch) rectangle. Cut squash into ¼-inch thick slices. (Reserve bulbs and trimmed sides for another use.)

Place a wire rack in a 12-inch skillet. Add water and bring to a boil over high. Working in batches, place squash planks in a single layer on wire rack, making sure all squash is directly over steaming water; cover and cook until tender-crisp, 6 to 8 minutes. Transfer squash to a large plate. Repeat process with remaining squash, adding water as needed.

While squash is cooking, stir together curry paste, 2 teaspoons of the oil, and ½ teaspoon of the fish sauce in a small bowl. Evenly brush cooked squash planks on both sides with curry paste mixture. Rinse skillet and rack; wipe dry and set aside.

Heat remaining 2 tablespoons oil in reserved skillet over medium. Working in batches, add squash planks to oil; cook, turning halfway through, until golden brown on both sides, about 2 minutes. Transfer squash to a reserved rack.

Stir together coconut milk, brown sugar, jalapeño, lime zest and juice, ginger, salt, and remaining 1 teaspoon fish sauce in a small saucepan until combined. Bring coconut milk mixture to a boil over medium high. Reduce heat to medium; simmer until fragrant and slightly thickened, about 2 minutes.

Transfer sauce to a wide shallow bowl or rimmed plate; place squash planks on top of sauce. Garnish with chopped cilantro, pepitas, and jalapeño slices.