1 tablespoon unsalted butter, plus more for greasing the pan
1 large white or yellow onion, finely diced
1 1/2 teaspoons kosher salt, plus more for the onions
2 garlic cloves, smashed, peeled, and minced
1 teaspoon ground mustard
1 cup heavy cream
1/2 cup buttermilk
3 large eggs
1 teaspoon hot sauce
3 cups fresh corn kernels (from about 4 cobs)
3/4 cup grated sharp-as-possible cheddar
6 tablespoons cornmeal
1 1/2 teaspoons baking powder
Set a medium skillet over medium heat and add the butter. When it’s melted, add the onion and a pinch of salt. Sauté for 10 to 15 minutes, stirring occasionally, until soft. Turn off the heat.
Add the garlic and mustard, and stir. Let cool while you tend to the rest of the recipe.
Heat the oven to 350° F. Butter a 9×9 inch baking dish (or other 1 1/2 quart–sized baking dish).
Combine the cream, buttermilk, eggs, 1 1/2 teaspoons salt, and hot sauce in a bowl or measuring cup. Whisk with a fork until smooth.
Combine the corn kernels, cheddar, cornmeal, and baking powder in another, larger bowl. Stir with a spoon or rubber spatula until combined.
Add the cooled onion mixture and stir. Add the liquid mixture and stir.
Pour into the prepared baking dish. Bake for about 45 minutes until the casserole has puffed up, is deeply browned along the edges, and browning on top. Cool for a few minutes before serving.