Put the acorn squashes in the oven on a baking sheet, whole, at 425 F degrees for 1 hour. When you take them out, cut them in half and let them cool to the touch while you start making the soup.
Saute the onion, carrot, celery and potatoes in the butter with a few big pinches of sea salt to bring out their juices and sweeten. Cook on medium heat for about 10 minutes.
While the veggies are cooking, scoop out the squash from the shell and puree the flesh in your processor of blender with a little of the bone broth to be sure it gets fully pureed.
Add the garlic to the pot and cook for a minute. Add the squash puree, bone broth, chicken and cayenne to the pot and simmer for about 5 minutes. Add the spinach to wilt and then season with sea salt and pepper to taste.
Serve the stew to baby on a spoon. If baby isn't handling bits of soft vegetables yet, you can puree his or her portion so it is smooth or semi-smooth.