I LOVE this recipe!
In a large, heavy skillet, sauté the onion in the olive oil until it begins to soften. Add the butternut squash and continue cooking over medium heat, stirring often until the squash is tender and cooked through (about 10-12 minutes). Reduce the heat and mash the squash/onion mixture with a potato smasher or the back of a large fork to your desired consistency. I prefer mine a bit coarse. Top with the fresh sage or rosemary and serve.