Pea Soup, minted with Smoked Salmon

Ingredients

• 3 tablespoons unsalted butter
• 1 onion, coarsely chopped
• 1 pound frozen peas, thawed
• 2 tablespoons coarsely chopped mint
• 3 cups chicken stock or low-sodium broth
• Kosher salt
• 1/2 pound Scottish smoked salmon in one piece, skinned and cut into 1/2-inch dice
• 2 tablespoons creme fraiche