Rinse squash and, using a metal skewer, pierce through top to center in 2 or 3 places. For easy handling, set squash on a piece of cheesecloth large enough to enclose it. Tie cheesecloth loosely on top of squash (leaving some access to vegetable for testing). Set squash on a rack above water 3/4 to 1 in. deep in a 5- to 6-qt. pan at least 1 in. wider than squash; water should not touch squash.
Cover and bring to a boil over high heat; steam over medium heat until very tender when pierced, about 30 minutes.
Protecting your hands, use cheesecloth to lift squash from pan; untie cloth. Neatly slice off top quarter of squash to make a lid. With a small spoon, gently scoop out seeds and discard; take care not to tear or poke a hole in shell (some kinds are more fragile, so handle carefully).
Fill squash with spinach, sprinkle with lemon zest, and set lid in place if you like. To serve, lift lid and scoop squash from the shell as you spoon out spinach.
Make ahead: After step 3, put lid on squash, wrap airtight, and chill up to 1 day. To reheat, place unwrapped squash on a microwave-safe plate in a microwave oven on full power (100%) for 4-minute intervals until hot, about 12 minutes. Or seal in foil and place in a 375° oven until hot, about 45 minutes; unwrap on a serving plate. Continue with step