Ingredients

1. 4 medium onions, peeled and quartered
2. 2 sticks (1/2 pound) unsalted butter, melted and cooled
3. 3/4 cup sugar
4. 4 extra-large eggs, lightly beaten
5. 1 tablespoon kosher salt
6. 1 tablespoon baking powder
7. 3 cups walnuts, coarsely chopped (3/4 pound)
8. 3 cups all-purpose flour

Directions

1. Preheat the oven to 425° and spray two 12-cup muffin tins with nonstick vegetable cooking spray.
2. 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. 
3. Let the muffins cool in the pan for 10 minutes, then unmold them on a wire rack and let cool. 

Make Ahead

The muffins can be frozen for up to 1 week. Thaw completely and rewarm before serving.