- Preheat oven to 350°. In a medium bowl, whisk together eggs, milk, ½ teaspoon sea salt and pepper. Add shredded cheese and mix. Set aside.
- Heat oven-proof non-stick skillet or cast-iron pan on medium-high. Add olive oil to warm and add diced onion. Sauté onions for 3 minutes while adding diced Swiss chard stems to soften. Add shredded Swiss chard leaves and sauté for 2 minutes to wilt. Add minced garlic and cook for an additional 1 minute.
- Reduce heat to medium-low and add egg mixture to sautéed onions and wilted greens. Using tongs, arrange greens evenly throughout the pan. Cook eggs in pan about 3-4 minutes until they are set before placing the pan in the oven to bake for 5 minutes.
- For a browned, crispy top, place the frittata under the broiler for a minute or two at the end of cooking.
- Cool in the pan for 5 minutes, then slice into wedges and serve.