Took about 25 mins total; use silicon cover; cheese will be baked on the bowl.
Cooking spray, for baking dish
2 pounds Yukon Gold potatoes sliced (thin blade on mandolin)
1 (10 3/4-ounce) can cream of mushroom soup
1 1/2 cups milk
2 cups shredded cheddar cheese
Using cooking spray, coat a 9 x 13-inch baking dish or a 2-quart gratin dish that fits in your microwave.
Peel the potatoes.
Use a knife or food processor fitted with the slicing blade to slice the potatoes 1/2-inch thick.
Stir together the cream of mushroom soup and milk in a large bowl.
Add the potatoes and stir to coat well.
Pour into the prepared pan.
Cover loosely with plastic wrap, folding back a corner to allow steam to escape. Heat in the microwave on high for 5 minutes. Rotate the baking dish and heat on high for another 5 minutes. Stir well.
Top with the cheese. Heat another 10 minutes or until the potatoes are fork-tender.
Tips
Make sure to slice the potatoes thinly, or they won't cook quickly enough.
Microwaves cook differently from each other and tend to cook unevenly. Make sure to check the doneness of the potatoes at each interval and stir the mixture if necessary to redistribute.
Recipe Variations
If you'd rather not use canned cream of soup, you can make a homemade version using butter, flour, chicken broth, and milk. (If you'd like to keep the recipe vegetarian, simply replace the chicken broth with vegetable broth or more milk.)
Feel free to change up the type of cheese, or use a combination of cheeses. Gruyère or Swiss are great choices, and Parmesan along with the cheddar is a nice blend of flavors.
A bit of fresh or dried herb will add a welcome earthiness to the recipe. Try thyme, sage, or rosemary.
For a more filling dish, add some cooked bacon or ham to the casserole.
https://www.thespruceeats.com/microwaved-scalloped-potatoes-recipe-2098665