Preheat the oven to 350°F. Butter a 2-quart or 9-inch square baking dish.
Melt in a large saucepan over medium heat:
Add and cook, stirring, until translucent, about 5 minutes:
Stir in and cook for 1 minute:
Add and bring to a boil:
Gradually whisk in:
Bring to a rapid boil, then remove from the heat. Stir in:
Add and stir to combine:
Pour the mixture into the prepared baking dish. Bake until firm and golden brown on top, about 1 hour.