Slow Cooker Butternut Squash and Sweet Potato Soup

medium yellow onion, chopped

medium-large butternut squash, peeled, seeded and chopped into 1-inch pieces

medium sweet potatoes (about 1 ½ pounds), peeled and chopped into 1-inch pieces

apple, cored and cut into 1-inch pieces (apple is optional – see note below*)

ground cinnamon

nutmeg

salt

black pepper,

low sodium vegetable broth**