Heat oil in a large saucepan over medium heat. Add leek and cook, stirring occasionally, 8 minutes, until softened. Stir in garlic and cook 1 minute.
Add stock and bring to a boil. Reduce heat to a simmer and cook, covered, 5 minutes. Add peas and return to a boil, then reduce and cook, covered, 5 minutes. Remove from heat and allow to cool slightly, about 10 minutes.
Add mint and 1/2 cup yogurt. Use a handheld stick blender to puree, or puree in batches in a food processor or blender. Return soup to pan if using a food processor. Season well with pepper and heat gently until hot but not boiling.
Ladle soup into serving bowls, top each with 1 tablespoon yogurt and a light sprinkling of sumac or paprika, and serve.
If using bought salt-reduced stock, use half stock and half water to keep the salt content at a healthy level.
Frozen vegetables are convenient yet just as nutritious as fresh.
Sumac is a dried berry that is ground and used in Middle Eastern dishes to add a lemony flavour.