Preheat oven to 350 degrees F (175 degrees C).
Place potatoes into a large pot and cover with salted water; bring to a boil. Reduce heat to medium-low and simmer until slightly tender, about 5 minutes. Drain.
Heat olive oil in a skillet over medium heat; cook and stir onion until lightly browned and tender, 10 to 15 minutes.
Mix onion, cream of mushroom soup, Cheddar cheese, and evaporated milk in a bowl. Arrange potatoes in a 9×13-inch casserole dish. Pour sauce over potatoes.
Bake in the preheated oven until potatoes are tender and sauce is bubbling, 20 minutes.
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