Vegetarian Skillet Pot Pie

Name: Vegetarian Skillet Pot Pie
Ingredients: 1 sheet frozen puff pastry (from a 17-ounce box), thawed; 1 small yellow onion; 1 pound sweet potatoes (about 2 medium); 1 pound cremini mushrooms; 2 cloves garlic; 1 teaspoon fresh thyme leaves; 5 ounces baby kale (about 5 packed cups); 3 tablespoons olive oil, divided; 1 teaspoon kosher salt, divided; 1/2 teaspoon freshly ground black pepper, divided; 3 tablespoon all-purpose flour; 2 cups low-sodium vegetable broth; 1/2 cup whole or 2% milk