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