In a mixing bowl, whisk together the condensed cream of mushroom soup, evaporated milk, and sour cream until smooth and well combined. Taste and adjust seasoning if needed. This creamy sauce is what binds all the layers together and creates that irresistible, gooey texture.
Step 4: Layer the Casserole
In your prepared baking dish, arrange a single layer of sliced potatoes on the bottom. Season lightly with salt and pepper. Spread about one-third of the cooked beef and onion mixture over the potato layer. Sprinkle about one-quarter of the shredded cheddar cheese over the beef. Repeat the layers — potatoes, beef, cheese — until all ingredients are used, finishing with a final layer of potatoes on top. Pour the creamy soup mixture evenly over the entire casserole, making sure it seeps down into all the layers.
Step 5: Bake Covered
Cover the baking dish tightly with aluminum foil and place in the preheated oven. Bake covered for 60–70 minutes, or until the potatoes are fork-tender. Check doneness by inserting a fork or skewer into the center — it should slide through the potato layers without resistance.
