Turn the oven on to 400 degrees F (200 C). Spray a large oven-safe baking dish with cooking spray so the potatoes won't stick. Set the pan aside while you prepare the vegetable scalloped potatoes.{"smallUrl":"https:\/\/www.wikihow.com\/images\/6\/63\/Make-Healthy-Scalloped-Potatoes-Step-9-Version-3.jpg","bigUrl":"\/images\/thumb\/6\/63\/Make-Healthy-Scalloped-Potatoes-Step-9-Version-3.jpg\/v4-190px-Make-Healthy-Scalloped-Potatoes-Step-9-Version-3.jpg","smallWidth":460,"smallHeight":307,"bigWidth":"190","bigHeight":"127","licensing":"<div class=\"mw-parser-output\"><p>License: <a rel=\"nofollow\" class=\"external text\" href=\"https:\/\/creativecommons.org\/licenses\/by-nc-sa\/3.0\/\">Creative Commons<\/a><br>\n<\/p><p><br \/>\n<\/p><\/div>"} Scrub 6 medium Yukon gold potatoes under running water. Peel the potatoes and use a sharp knife to slice them about 1/4-inch (6 mm) thin. Set the potatoes aside while you prepare the other vegetables and the sauce. If you have a mandolin, use it to quickly and evenly slice the potatoes. Melt 2 to 3 tablespoons of butter in a large saucepan or pot over medium heat. Wash 1 leek really well to remove any sand or grit and slice it thinly. Add it to the pan along with 2 cups (150 g) of cremini mushrooms that you've cut into quarters. Stir the vegetables and cook them for 2 to 3 minutes. Sprinkle them with a little salt and pepper. To ensure the leek has been washed well, consider slicing it in half and submerging the halves in clean water. Swish the leek around to loosen any sand or grit. Keep the heat on medium and add 3 cloves of minced garlic, ¼ teaspoon of ground nutmeg, and 1 teaspoon of garlic powder. Stir the vegetables and cook them for 2 minutes. Add ¼ cup (29 g) of spelt flour. Stir and cook the mixture for a minute or two. Slowly stir in 1 ½ cups (354 ml) of almond, soy, or skim milk. Continue to stir and cook the vegetable sauce until it thickens a little. This should take several minutes. If you add the milk too quickly, the sauce may become lumpy. If it does, whisk the mixture to break up the lumps. Turn the heat off and add ¼ cup (70 g) of Greek yogurt. Stir the sauce so the yogurt is completely incorporated. You can use either whole-milk or skim Greek yogurt, just stir it well before you measure and add it to the sauce. Spread one layer of the sliced potatoes on the bottom of the prepared baking dish. The potato slices should overlap a little. Pour 2 to 3 large scoops of the vegetable sauce over the potatoes. Spread a layer of the sliced onion (from 1 medium onion) over the sauce. Repeat the layers, so you end up with potatoes on the very top. Sprinkle the final layer of potatoes with 1/2 teaspoon of sea salt, 1/4 teaspoon of ground black pepper, and 1/4 teaspoon of paprika. Tear off a sheet of aluminum foil and cover the baking pan. Put the casserole in the oven and bake it for 45 minutes. Carefully remove the pan and take the foil off. Return the pan to the oven and bake it for 15 more minutes. The potatoes should be completely soft when you poke them with a fork. Turn the broiler on and place the scalloped potatoes a few inches below the element. Broil the potatoes for 3 to 5 minutes. The top should become dark brown and crisp. Remove the pan and let the potatoes rest for 5 minutes before you serve it.  Consider garnishing the scalloped potatoes with sliced green onions. You can store leftovers in an airtight container in the refrigerator for up to 5 days.

Summary:
Preheat the oven and prepare a baking dish. Prepare the potatoes. Saute the mushrooms and leek. Add and saute the garlic and seasonings. Stir in the spelt flour and milk. Stir in the Greek yogurt. Assemble the scalloped potatoes. Season the top of the potatoes and cover the dish. Bake the casserole. Broil and serve the vegetable scalloped potatoes.