Heat oil in a 10-inch cast-iron skillet over medium heat. Add hash browns; sprinkle with Greek seasoning, and sauté 4 to 5 minutes or until lightly browned. Add chopped artichokes, and cook 2 minutes or until thoroughly heated. Stir in spinach, and cook 2 minutes or until spinach wilts, stirring frequently.
Separate eggs, reserving 4 egg yolks for another use. Combine remaining 2 egg yolks, whites, and milk, stirring with a whisk. Pour egg mixture over potato mixture. Sprinkle with pepper and feta cheese. Arrange tomato slices over feta. Cook over medium heat 3 to 4 minutes or until edges are set. Place frittata under broiler, and cook 4 to 5 minutes or until center is set and top begins to brown. Cut into 8 wedges.