How to Make It
Preheat the oven to 425° and spray two 12-cup muffin tins with nonstick vegetable cooking spray.
In a food processor, pulse the onions until pureed. Transfer 2 cups of the onion puree to a bowl and stir in the butter, sugar and eggs. One at a time, stir in the salt, baking powder, walnuts and flour; mix thoroughly. Spoon the batter into the prepared tins and bake for 20 minutes, or until the muffins are brown and a toothpick inserted in the center comes out clean.
Let the muffins cool in the pan for 10 minutes, then unmold them on a wire rack and let cool.