Turkey Shepherd's Pie with Butternut Squash
1 butternut squash, halved lengthwise 1 clove garlic, chopped 1 medium onion 1 pound ground turkey, or more to taste 1 sweet potatoes, diced 1 tablespoon butter 1 tablespoon extra-virgin olive oil, divided 1 teaspoon Worcestershire sauce 2 teaspoons milk, or to taste salt and ground black pepper to taste ¼ teaspoon chili powder, or to taste ¼ teaspoon dried oregano, or to taste ¼ teaspoon dried thyme, or to taste ½ cup frozen corn ½ cup frozen peas ½ cup turkey gravy