1.Preheat oven to 400°F. Spray a 9 x 11-inch glass baking dish with nonstick cooking spray.
2.Wash and dry greens. Separate leaves from stems and chop stems into 1-inch pieces. Chop leaves into 2-inch pieces. Set aside.
3.Heat 1 teaspoon vegetable oil in skillet over medium heat. Add the
onion and cook until lightly brown, about 5 minutes.
4.Add the stems, salt, pepper, and nutmeg. Cover and cook, stirring occasionally, until tender, about 5 to 7 minutes.
5.Add leaves a handful at a time, mixing as you go. Cook until all leaves are wilted and combined. Remove from heat.
6.Heat 2 teaspoons vegetable oil in a saucepan over medium heat. Add the garlic and cook for 30 seconds to 1 minute. Whisk in the flour and then slowly whisk in the milk.
7.Stir until sauce is thick enough to coat the back of a spoon, about 2 minutes. Remove from heat and set aside.
8.Combine cheeses in a bowl. Arrange half of the sweet potato slices in a single layer in the bottom of the prepared baking dish. Sprinkle with oregano. Add half of the collard greens mixture. Sprinkle with half of the cheese. Pour half of the sauce mixture over the top. Add a second layer of potato slices, then collard greens, then remaining sauce. Sprinkle the remaining cheese over the top.
9.Bake at 400°F for 40 to 45 minutes uncovered, until potatoes are cooked through. If the cheese starts to brown too much before the cooking time is over, cover it loosely with foil.
10.Remove dish from oven and serve hot.