Chop onions and mushrooms (can be done up to 3 days ahead).2.
Whisk eggs with milk and some salt and pepper. Stir in feta.
Preheat oven to 400F degrees.2.
Heat up a skillet over medium heat. Add in cooking oil and then onions to heated oil with a dash of salt. Saute for about 5 minutes, until softened. Next add in mushrooms and cook for another 3 to 5 minutes, to let mushrooms soften.3.
Add in kale leaves and toss through to let the heat wilt the kale. Pour in egg and feta mixture. Stir to mix everything through and then transfer to oven. Cook for 18 to 22 minutes until knife comes out cleanly.