1 (2 pound) butternut squash - peeled, seeded, and cut into large chunks
4 sweet potatoes, peeled and cut into chunks
1 onion, peeled and chopped
1 (2 inch) piece fresh ginger, peeled and finely chopped
3 cloves garlic, peeled and chopped
1 1/2 quarts water, or amount to cover
1 cup plain yogurt, or amount desired (optional)
salt and ground black pepper to taste