Preheat oven to 400° Fahrenheit (200° Celsius).

Transfer peeled and diced potatoes into a pot and pour water in to submerge them. Add salt (2 teaspoons per 3 cups of water). Cover and bring to a boil, then reduce heat and simmer until almost done.

Heat oil in a large frying pan, add onion and sauté until translucent, about 4 minutes. Add spinach in stages and sauté until wilted. Stir in garlic and cook until aromatic. Turn off heat. Mix sour cream, salt, black pepper, oregano, and garlic powder until smooth.

Drain potatoes and combine with spinach and sour cream mixture in an oven-proof dish. Cover with grated cheese.

Cook in the oven until cheese is golden brown and potatoes are soft, about 25 minutes.