Creamy Butternut Squash Polenta

Bahan-bahan (4 porsi)

• 5 tablespoons unsalted butter, (divided)
• 1 cup very finely chopped yellow onion, (from one small onion)
• 4½ cups whole milk
• 1 (12-oz) package frozen butternut squash or winter squash purée (about 1½ cups), thawed (see note)
• 1½ teaspoons salt
• ¼ teaspoon freshly ground black pepper
• ¾ cup instant polenta or fine yellow cornmeal
• ¼ cup finely grated Parmigiano-Reggiano or Pecorino Romano
• Finely chopped fresh herbs, (for serving (optional))