Coat squash lightly with olive oil and place on baking sheet and place under broiler for about 10 minutes. Place 5-6 inches below broiler. Flip half way through cooking time.
Melt butter in large pan over medium heat and cook onion, garlic and thyme until onion is softened.
Add in roasted squash and chicken broth. Cook until squash is tender. About 10-15 minutes.
Add in cumin, allspice and salt and pepper.
Remove from heat and puree in blender to desired consistency. Depending on the type of blender you have, you may need to let the soup cool first.
Notes
Please be extra-careful when blending hot liquids in a traditional blender. I recommend letting the soup cool a while if you are not using a high speed blender and puree the soup in 2-3 batches, not overfilling the blender. Then also be sure to slightly tent the cap of the blender lid so that the extra steam can escape or else you will have a big mess!