12ounces(3 cups) shredded white Cheddar and/or pepper Jack,
6slicescooked bacon, crumbled
On high heat, bring a 4-quart pot of water to a boil. As the water is heating, trim the Brussels sprouts. First, thinly slice off the woody bottom of each sprout, then either cut the sprout in half, if the circumference of the sprout is less than an inch wide, or into quarters, if the circumference is larger than an inch wide.
When the pot of water is boiling, leaving the heat on high, add the salt and the Brussels sprouts to the pot and cook for 5-7 minutes or until tender. Drain the Brussels sprouts in a colander and rinse with cold water to stop the cooking.
Preheat the broiler and place a rack 6 inches away from the heat.
In a 10-inch cast-iron skillet, melt the butter on low heat and add the jalapeños. While occasionally stirring, cook the jalapeños until lightly softened, about 2-3 minutes. Add the garlic and cook for 30 more seconds. Whisk in the flour and cook until lightly browned and fragrant, about 1 minute. Pour in the milk and whisk until everything is well combined. While occasionally stirring, cook until the milk has thickened, about 5 minutes. Be sure not to let it come to a boil.
Turn off the heat and stir half of the cheese into the sauce until it’s melted. Add the cumin, and cayenne, then taste and adjust seasonings, adding black pepper and salt to taste. Stir in the Brussels sprouts and crumbled bacon until well combined. Evenly top the sprouts with the remaining cheese and place under the broiler for 1 1/2 to 3 minutes or until the top is browned. (My broiler takes 2 minutes but yours may take more or less time so keep an eye on it so it doesn’t burn.)
Alternatively, if you don’t have a cast-iron or broiler-proof skillet, pour the gratin into lightly greased 2 1/2-quart broiler-proof baking dish and broil it that way.
Allow the gratin to rest for 10 minutes before serving. Serve warm.