Rinse squash and cut each in half lengthwise; scoop out and discard seeds. Place squash halves, cut side down, in a 10- by 15-inch pan.
Bake in a 400° regular or convection oven until squash is just tender when pierced, 25 to 30 minutes.
Meanwhile, squeeze moisture from spinach. In a bowl, mix spinach, 3/4 cup cheese, and walnuts. Season to taste with salt and pepper.
With tongs or a spatula, turn squash halves over and spoon spinach mixture equally into cavities. Sprinkle equally with remaining 1/4 cup cheese. Return squash to oven and bake until cheese melts and filling is hot in the center, 15 to 20 minutes.