Preheat oven to 180C (350F). Grease or line muffin tin.
In a large bowl, sift together flour, baking powder, baking soda and salt.
In another bowl, whisk together, sugar and egg until creamy. Stir in oil, yogurt, lemon zest and banana. Combine wet ingredients with dry.
In a food processor, blend the spinach leaves until finely chopped.
Fold chopped spinach and walnuts into the batter, leaving some of the walnut aside. Fill batter into prepared muffin tin. Garnish each muffin with the remaining walnuts. Bake for 20 minutes or until toothpick comes out clean (11-12 min for mini muffins).