01 - Melt the butter in a large saucepan over medium heat. Add the chopped onion and sliced leek, sautéing for 4-5 minutes until softened but not browned.
02 - Stir in the minced garlic and cook for 1 minute until fragrant, being careful not to burn it.
03 - Add the shelled peas and vegetable broth to the saucepan. Bring to a boil, then reduce heat to low. Simmer for 8-10 minutes for fresh peas or 5 minutes for frozen peas, until tender and bright green.
04 - Remove the saucepan from heat. Stir in the fresh mint leaves and let them steep briefly in the hot soup.
05 - Using an immersion blender, purée the soup directly in the saucepan until completely smooth. Alternatively, transfer to a countertop blender in batches, being cautious with hot liquids.
06 - Season the soup with sea salt and freshly ground black pepper. Taste and adjust seasoning as needed to balance flavors.
07 - Ladle the warm soup into serving bowls. Top each portion with a dollop of crème fraîche, a sprinkle of lemon zest, and additional fresh mint leaves. Serve immediately while hot.