Smoky Scalloped Potatoes

These are the scalloped potatoes I brought to this evening's pig roast. They're from an America's Test Kitchen recipe. I like to add a bit of synthetic hickory smoke, but the potatoes are delicious without it.

4 tablespoons unsalted butter
1 cup minced onion [I like to use a bit more]
4 garlic cloves, minced [I use 6-8, depending on size]
4 teaspoons powdered mustard
1 tablespoon minced fresh thyme [dried thyme works fine, if you don't have fresh on hand - the rule of thumb is 1/3 the fresh quantity, but I use somewhat more...]
2 1/2 teaspoons salt
1/2 teaspoon cayenne pepper
[1-2 teaspoons liquid hickory smoke flavor]
5 lbs russet potatoes, peeled and sliced thin
1 1/2 cups buttermilk
1 1/2 cups heavy cream
1/4 teaspoon baking soda
8 ounces smoked Gouda cheese, shredded

1. Heat the oven to 425 degrees Fahrenheit. Melt the butter in a Dutch oven [any thick-bottomed pot works fine] over medium-high heat. Add the onion and cook until softened, about 5 minutes. Add the garlic, mustard, thyme, salt, and cayenne and cook until fragrant, about 30 seconds. Stir in the potatoes, cream, buttermilk, baking soda, and liquid smoke and bring to a simmer. Reduce the heat to medium-low, cover, and cook until the potatoes are almost tender, about 15 minutes. Stir in the cheese and transfer the mixture to a 13 x 9" baking dish.

2. Bake until the cream is bubbling around the edges and the top is golden brown, about 15 minutes.

Do ahead option: You can prepare the potatoes through step 1 and refrigerate, covered with foil, for up to 25 hours. Bake covered at 400 degrees until hot and bubbly, about 40 minutes. Remove the foil and continue cooking until the top is golden brown, another 30 minutes.