Preheat the oven to 350°F (180°C). Grease a 12-cup muffin pan or lightly oil paper liners.
In a large bowl, mash 2 ripe bananas until mostly smooth. Add 1 large egg, ½ cup Greek yogurt, ⅓ cup honey, and 2 teaspoons vanilla extract. Whisk until smooth.
Add 1½ cups zucchini (grated, no need to squeeze it), ½ cup raisins, and ⅓ cup walnuts. Stir to combine.
Add 1 cup rolled oats, 1½ cups all-purpose flour, 2 teaspoons cinnamon, ¼ teaspoon nutmeg, 2 teaspoons baking powder, ½ teaspoon baking soda, and 1 pinch salt. Stir gently until just combined. Do not overmix.
Divide the batter among 12 muffin cups, filling them almost to the top.
Bake for 20 to 22 minutes, or until a toothpick inserted in the center comes out mostly clean.
Let the muffins cool in the pan for 5 minutes, then transfer them to a wire rack to cool completely.
