Scalloped potatoes are like a casserole, as sliced potatoes are layered and baked. Typically, their silky texture and flavor come from milk and a roux of flour and butter.
Cream of mushroom soup reduces the required ingredients and adds tons of flavor to the dish. You may still want to add some butter for richness, but the soup does most of the work.
The soup's additional ingredients, like onions, chicken broth, and cheese, complement the potatoes. It adds a salty, earthy flavor that's enhanced with grilled mushrooms.
Russet potatoes are a good option for this dish due to their starch and moisture content. If you use cream of mushroom soup, opt for Gruyère cheese for a subtle, nutty flavor.