Melt butter in 10 inch nonstick skillet over medium heat. Cover and cook potatoes in butter about 8 minutes, stirring occasionally, until potatoes are tender. Beat eggs, milk, marjoram and 1/4 tsp salt and set aside. When potatoes are done, stir in spinach, tomatoes, and onions. Cook, stirring occasionally, just until spinach is wilted. Reduce heat to medium low. Pour in egg mixture. Cover and cook about 6 minutes or just until top is set. Sprinkle with cheese. Cover and cook about 1 minute or until cheese is melted.