Prepare Squash: Carefully, with a sharp knife, slice squash in half lengthwise (from stem down to bottom). If you are having trouble, poke squash with fork several times and place in microwave to cook in 30-second increments to slightly soften to make cutting easier. Then, using a spoon, scrape out seeds from squash halves. Season halves with salt and pepper.
Pour water into Ninja Foodi. Transfer squash halves in Cook & Crisp Basket, cut side down. Place basket inside pot.
Close lid and move slider to “STEAMCRISP” setting.
Cook Squash: Select “STEAM & CRISP” setting, set temperature to 390 degrees, and set time to 15 minutes. Press “START/STOP” to begin cooking (the screen will display “PrE” for ~7-8 minutes as the unit steams, then 15-minute timer will begin counting down).
Prepare Sweet Butter: While squash cooks, prepare sweetened butter by whisking together melted butter, monk fruit sweetener, and pumpkin pie spice. Set aside.
Final Steps: Open lid, using a tongs, remove squash halves from Cook & Crisp Basket and set on a plate. Using a pastry bush, brush on sweetened butter and sprinkle on salt and pepper. Serve.