Preheat the oven to 425 degrees F. Line a 12-cup muffin pan with paper liners; set aside. Melt the butter in a large skillet over medium-high heat. Add the leeks and celery and cook, stirring, until wilted, about 5 minutes. Add the parsley, thyme, walnuts and raisins; season with salt and pepper. Cook, stirring, until the vegetables are tender, about 5 more minutes; set aside to cool.
Whisk the cornmeal, flour, sugar, baking powder, baking soda and 1/2 teaspoon salt in a medium bowl. Whisk in the milk, sour cream and egg until combined. Stir in all but 1/2 cup of the cooled vegetable mixture. Divide the batter among the prepared muffin cups, filling each almost to the top. Sprinkle the reserved vegetable mixture on top. Bake until the muffins bounce back when pressed, about 15 minutes. Let cool 5 minutes in the pan, then remove to a rack to cool completely.