Preheat oven to 450ºF (with convection turned on, if you have it). Line two baking sheets with parchment paper. Divide sweet potato slices between baking sheets and drizzle with oil. Sprinkle nicely with salt and toss to coat evenly. Bake for about 20 minutes, flipping potatoes and rotating pans once. They're done when they've browned a bit on top and have crispy edges.

Meanwhile, make cheese sauce. Pour milk into a small saucepan off of heat. Whisk in cornstarch, then bring to a boil over medium heat, whisking frequently to avoid scorching the bottom. Let boil for a minute then remove from heat and stir in cheese. Add salt to taste.