Lightly grease the inside of your slow cooker with butter or cooking spray.
Arrange a layer of 4 cups thinly sliced potatoes and 1 cup thinly sliced onions in the bottom of the slow cooker.
Sprinkle a portion of 2 cups shredded cheddar cheese, 1 teaspoon garlic powder, 1 teaspoon onion powder, 1 teaspoon salt, and ½ teaspoon black pepper over the potatoes.
Continue layering potatoes, cheese, and seasonings until all ingredients are used, finishing with a layer of cheese on top.
Pour the 1 cup heavy cream evenly over the layered potatoes and dot the top with diced 3 tablespoons butter.
Cover and cook on low for 6–7 hours or high for 3–4 hours, until potatoes are tender and the sauce is creamy.
Sprinkle with 1 tablespoon fresh parsley or thyme before serving, if desired, for a fresh, vibrant finish.