Essentially, you can use any (except spaghetti squash) type of squash to make the puree, or even sweet potato. Just slice the squash in two, remove the seeds, peel off the skin and cube into 2 cm cubes. Make them smaller if you are in a hurry. Steam until fork tender, usually 10 minutes on the oven or in small batches in the microwave 2 to 4 minutes.
Once your squash is tender, throw it in the blender – careful, it's hot – and blend until pureed. You can spice it up with some ground cloves, cinnamon and nutmeg for that pumpkin spice goodness.