Preheat the oven to 425° F. Toss chopped butternut squash with balsamic vinegar, 2 tablespoons grapeseed oil, garlic powder, salt, and pepper. Spread evenly on baking sheet and
roast for 40 minutes. After 40 minutes, add the chopped portabella caps and toss with the squash. Bake for another 15 minutes.