Add the potatoes to a large pot and cover with cold water. Bring to a boil and then simmer until fork tender.
Drain the potatoes and mash with a potato mashed.
In the meantime, heat the cream, milk, butter, salt, and olive oil until warm.
Slowly add in the warm liquids while mashing.
You can optionally whip the mashed potatoes with a hand blender for a smoother texture (don't whip too much or the potatoes might become gluey).
Finally, fold in the grated Manchego cheese.
Taste for salt and adjust if necessary. Serve immediately.
To reheat, warm the mashed potatoes in the microwave and heat about 1/4 cup of whole milk separately. Drizzle in the whole milk little by little until the potatoes are at the desired consistency.