Creamy Scalloped Yukon Spuds

• 1 stick butter
• 1,500 gr thinly yukon potatoes (sliced)
• 240 ml milk
• 230 gr cheddar cheese (grated)
• 30 gr onion (finely chopped)
• 1 gr salt
• 1 gr pepper
• 1 package cream of mushroom soup