Broccoli Purée

Bahan-bahan (6 porsi)

• 4 tablespoons unsalted butter, (divided, plus more for serving if desired)
• ½ cup chopped yellow onion, (from 1 small onion)
• ¾ cup water
• 1 teaspoon salt, (plus more to taste)
• ½ teaspoon freshly ground black pepper
• 2 pounds broccoli crowns, (cut into florets, tough stems thinly sliced (see note))
• 1 tablespoon fresh thyme leaves
• ¼ cup sour cream
• 1 tablespoon finely chopped chives, (for serving)