Preheat oven to 375 F. Coat a 13 x 9 oven safe baking dish with 2 tbsp of butter and set aside.
Heat the remaining 2tbsp of butter in a large sauté pan over medium heat. Add onions and garlic and sauté stirring often, until softened and translucent about 2 minutes. Add red peppers and cook until softened for an additional 2-3 minutes. Stir in herbs and then add Kale. Cook for 1-2 minutes until kale has softened. Remove from heat and line the bottom of the baking sheet with kale mixture. Add 1/2 cup of the tomatoes and sprinkle a 1/4 cup of the feta on top.
Meanwhile, whisk eggs, yogurt and salt and pepper in a large bowl.
Pour the egg mixture over the vegetable mix in the baking pan and sprinkle the remaining 1/4 cup of feta on top.
Place frittata in the oven and bake for 20 minutes. Add remaining 1/2 cup of tomatoes gently on top and place back in oven for 30-35 additional minutes until frittata has set. Remove from oven and enjoy with a simple side salad.