The frittata should be just cooked through. Keep an eye on it so it doesn’t brown and over-cook. You can also check doneness by sticking a toothpick into the center. It should come out clean when done.

Preheat oven to 350 degrees. In a large bowl, lightly whisk eggs and milk; then stir in garlic and cheese.  Season with salt and black pepper and set aside. In a 10-inch, ovenproof skillet, heat olive oil over medium heat. Add onion and chard stems and sauté until tender, about 5 minutes. Add chard leaves and continue cooking for about 6 or 7 minutes, stirring occasionally. Stir in tomatoes. Cook vegetables until any liquid is evaporated. Make sure all the ingredients are evenly distributed across the bottom of the skillet. Carefully pour in the egg mixture so that it evenly coats everything; sprinkle in dill and additional cheese. Reduce the heat to medium-low, and cook, without stirring, until eggs have set and start to pull away from the pan, leaving only top layer uncooked, about 7 minutes. Transfer the skillet to the oven and cook until set, about 10-12 minutes longer, or until cooked.Remove from the oven and let cool briefly.  Flip frittata onto a plate if preferred before cutting into wedges. Serve immediately or at room temperature.