1. Remove and mince chard leaves and cut stems in ½-inch-long pieces.
  2. Maintain leaves and stems in separate containers, and set aside.
  3. Heat 1 tablespoon olive oil in 10-inch skillet over medium heat.
  4. Add butter, and when melted, toss in onions.
  5. Cook, stirring frequently, over medium heat, about 5 minutes, then reduce heat to low, add ½ teaspoon salt and cover skillet.
  6. Cook 20 minutes more, or until onions are soft.
  7. Stir in potatoes and herbs.
  8. Cover and cook about 10 minutes, stirring intermittently, or until potato slices are tender.
  9. Add chard stems and sauté about 2 minutes more.
  10. Stir in chard leaves and garlic and cook another minute, or until leaves are wilted but still bright green.
  11. Remove pan from heat.
  12. Break eggs into large bowl, add remaining ½ teaspoon salt, and beat well with whisk.
  13. Add vegetables, black pepper to taste and cheese.
  14. Stir until evenly distributed.
  15. Clean and dry skillet and return to burner over medium heat.
  16. Preheat broiler.
  17. When skillet is hot again, add remaining olive oil, wait about 30 seconds, and swirl to coat pan.
  18. Pour in vegetable-egg mixture and cook undisturbed over medium heat, 3 to 4 minutes, or until bottom of eggs have firmed.
  19. Transfer skillet to broiler, and broil about 3 minutes, or until frittata is firm in center.
  20. Remove from broiler, and run knife around edge to loosen frittata.
  21. Slide or invert onto large, round plate, and serve hot, warm or at room temperature, cut into wedges.
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