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