Preheat the oven to 375°F (190°C). Lightly grease the 9x13 inch baking dish with cooking spray or olive oil.
In a large bowl, toss the diced sweet potatoes with olive oil, ½ teaspoon salt, and black pepper until well coated.
Spread the sweet potatoes evenly in the baking dish. Bake in the preheated oven for 20 minutes, or until fork-tender.
In the same large mixing bowl, whisk together the eggs, milk, remaining salt, oregano, and garlic powder until fully combined.
After removing the sweet potatoes from the oven, layer the chopped greens and halved cherry tomatoes over the sweet potatoes. Pour the egg mixture evenly over the top.
If using feta cheese, sprinkle it on top of the casserole.
Return the baking dish to the oven and bake for an additional 25-30 minutes, or until the eggs are set and slightly golden on top.
Allow the casserole to cool for a few minutes before slicing and serving.