Heat a 12 inch cast iron pan over medium-high heat. Once hot, add two tablespoons olive oil. To the hot oil add sliced mushrooms and diced bell pepper. Saute until lightly browned, about 6-8 minutes.
Add in the butter, kale, garlic and ¼ salt. Saute until kale is wilted and a little crispy. Olive oil can be used in place of butter if desired, but I find that butter adds a great flavor.
Meanwhile, whisk together eggs, milk, ¼ teaspoon salt, pepper until thoroughly combined.
Once the kale is ready, turn the heat off and crumble goat cheese evenly across the veggies. Pour the egg mixture over the veggies, tilting the pan as needed to evenly distribute.
Bake uncovered at 400F for 14-16 minutes. You can tell the frittata is done because the edges will be golden and pulling away from the cast iron a bit. It should also puff up a bit (it's okay if it doesn't puff up, that just means a hole caused the air to escape). Test for doneness by inserting a toothpick in the center of the frittata. Remove from the oven and enjoy!
Notes
Kale: If using whole kale leaves, prep the kale by thoroughly rinsing and drying it. Remove the stem and slice the leaves into wide ribbons. Spinach can be used in place of kale if desired, but do note that it will wilt more quickly.Do I have to use cast iron? I prefer using cast iron, but a medium casserole dish will work. You will need to adjust time based on the size of the dish, so keep an eye while its cooking and test for doneness.Leftovers: Keep leftovers refrigerated for 3 to 4 days. Reheat slices in the microwave in 1-2 minutes.