01 - Melt butter in a large saucepan over medium heat. Add chopped onion and sliced leek, sautéing for 4-5 minutes until softened and translucent, ensuring they don't brown.
02 - Stir in minced garlic and cook for 1 minute until fragrant, being careful not to burn it.
03 - Add peas and vegetable broth to the pan. Bring to a boil, then reduce heat to low and simmer. Fresh peas need 8-10 minutes; frozen peas require 5 minutes. Cook until peas are tender and bright green.
04 - Remove saucepan from heat. Stir in fresh mint leaves and let the soup cool slightly for 2-3 minutes to allow flavors to meld.
05 - Using an immersion blender, purée the soup directly in the pan until completely smooth and velvety. For an even silkier texture, strain through a fine-mesh sieve.
06 - Season with sea salt and black pepper. Taste thoroughly and adjust seasoning as needed to achieve balanced flavor.
07 - Ladle hot soup into serving bowls. Top each portion with a dollop of crème fraîche, a sprinkle of lemon zest, and fresh mint leaves. Serve immediately while hot.