Preheat oven to 375 degrees.
Lay out squash on a sheet pan, cut side up. Drizzle with one tablespoon of the grapeseed oil. Sprinkle with salt and pepper to taste. Roast for 40-45 minutes.
Saute onions and garlic while the squash is roasting: add the remainder of the grapeseed oil to a skillet over medium heat. Saute onions and garlic, stirring occasionally, until onions are translucent. Remove from heat.
Allow the butternut to cool after roasting. Once it's cool enough to handle, scoop out the roasted flesh. Add to blender with onions, garlic, and broth (it may be necessary to blend in batches. You can also use an immersion blender). Blend until smooth; season with salt and pepper to taste.