Southwestern Au Gratin Potatoes

Preheat the oven to 425°F and spray a 9×13 baking dish with cooking spray. Set it aside. Add the cheeses to a medium-sized bowl and combine them together with a fork. Set that aside, too. 
In a medium saucepan over medium heat, melt the butter. Once melted add the sliced onion and jalapeno. Toss in the butter and cook, stirring occasionally until softened and pliable.
Next, sprinkle the flour over the onions and jalapenos. Stir to coat them in the flour and let them cook for 2-3 minutes, then add the minced garlic. Cook for 2 minutes more. 
Pour in the heavy cream, either gradually or in batches to avoid lumps. Once completely incorporated stir in all of the seasonings except the cayenne pepper followed by 2/3 of the cheese. 
Once the cheese has completely melted into the sauce, remove it from heat. Taste it for seasonings and adjust as you like. 
Spoon a small amount of sauce into the bottom of the prepared baking dish and smooth it around with the back of the spoon. Cover it completely with 1/3 of the sliced potatoes, as evenly as you can. Ladle more sauce over the potatoes in thin ribbons, smooth that out and repeat the potato layer with another 1/3. Repeat these steps one more time and top it with all of the remaining sauce. 
Spread the remaining cheese across the top evenly, as best you can. It’s OK if there are a few bare spots, it’ll make for a delicious texture. Sprinkle to cayenne pepper over the top.
Cover tightly with aluminum foil and place in the oven on the middle rack for 25 minutes, then remove the foil and return to the oven for another 25-30 minutes until browned and bubbling. 
Let rest at room temperature for at least 15 minutes before serving. It will barely cool, but it will give it time to congeal a bit so it doesn’t run all over the place when you’re attempting to serve it.