Preheat the oven to 350°F (175°C). Grease a 2-quart casserole dish.
STEP 2
Place potatoes in a large pot, cover with salted water, and bring to a boil. Cook until just tender, about 15 to 18 minutes. Drain and cool before peeling. Slice thinly and transfer to a large bowl.
STEP 3
In a saucepan over medium heat, heat milk. Gradually whisk in flour, salt, pepper, and garlic powder until smooth. Cook until boiling and thickened. Add Cheddar cheese and jalapeños; stir until melted. Pour sauce over potatoes and scatter pimentos on top. Transfer to the prepared dish.
STEP 4
Bake in the preheated oven until potatoes are tender, about 30 minutes.