Creamy Onion Tart

Preheat the oven to 325°F.

In a large skillet, combine the onion, 2 tablespoons of the butter, and a generous pinch of salt. Cover and cook, stirring occasionally, over medium-low heat until very tender and pale gold, about 25 minutes.

In a large saucepan, melt the remaining 2 tablespoons butter over medium-low heat. Add the flour and cook, whisking constantly, until the mixture smells nutty, about 2 minutes. Whisk in the milk and cream in a slow, steady stream. Still whisking, bring to a boil and cook for 2 minutes. Remove from the heat and let cool slightly, whisking occasionally.