2 sticks (1/2 pound) unsalted butter, melted and cooled
3/4 cup sugar
4 extra-large eggs, lightly beaten
1 tablespoon kosher salt
1 tablespoon baking powder
3 cups walnuts, coarsely chopped (3/4 pound)
3 cups all-purpose flour
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.
The muffins can be frozen for up to 1 week. Thaw completely and rewarm before serving.