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, taking care not to brown the vegetables.
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 saucepan. Bring to a boil, then reduce heat to maintain a gentle simmer. Cook for 8-10 minutes for fresh peas or 5 minutes for frozen peas until tender and bright green.
04 - Remove saucepan from heat. Stir in fresh mint leaves and allow to steep for 2-3 minutes.
05 - Using an immersion blender, purée the soup directly in the saucepan until completely smooth. Alternatively, work in batches using a countertop blender, filling no more than two-thirds full and venting lid to release steam.
06 - Season with salt and pepper to taste. For an exceptionally silky texture, strain soup through a fine-mesh sieve, pressing solids with a ladle to extract all liquid.
07 - Ladle hot soup into warmed serving bowls. Top each portion with a generous dollop of crème fraîche, a sprinkle of lemon zest if using, and additional fresh mint leaves. Serve immediately while hot.