Minty Pea Soup

INGREDIENTS

• 3 tablespoons unsalted butter
• 1 medium onion, chopped
• 4 cups low-sodium vegetable broth, divided
• 6 cups shelled fresh peas (from about 6 pounds pods) or frozen peas, thawed
• 1/4 cup fresh flat-leaf parsley leaves
• 1/4 cup fresh mint leaves
• Kosher salt, freshly ground pepper
• 1/4 cup crème fraîche or sour cream
• 2 tablespoons heavy cream or water
• Chopped fresh chives (for serving)