Turn the oven on to 400 degrees F.
Take out a sheet pan with edges and place a piece of parchment paper on the bottom.
Place the cherry tomatoes on the sheet pan and pour one tablespoon of oil (avocado or olive) and 1/4 teaspoon of kosher salt over the tomatoes.
Mix the tomatoes, oil, and salt together with your hands to cover the tomatoes. Put the tomatoes in the oven for 20 minutes.
Crack all the eggs into a large bowl and discard the shells. Measure out 1/2 teaspoon of the kosher salt, the ground pepper, and the garlic salt with measuring spoons and dump them into the bowl with the eggs. If using milk, pour into the bowl now (you can also use cream or coconut milk). With a whisk or fork, stir the eggs until well combined. Set aside.
Place a cast iron skillet on the stove over medium heat. Place 1 tablespoon of oil (avocado or olive) and let heat up while you chop the onion.
Using a cutting board and large sharp knife, dice the onion. Then place in the skillet, add a pinch of salt, and cook for 5 minutes, stirring often with a wooden spoon or spatula until soft and mostly translucent.
While the onion is cooking, wash the kale and tear it away from the hard stem. On the cutting board cut the kale into large pieces with the large sharp knife. When the onion has cooked for 5 minutes, add the kale to the skillet to cook until it has wilted and cooked down, 2 to 3 minutes.
If the tomatoes aren’t done cooking yet, turn the burner heat off and remove the skillet from the burner. Once they are done, place the skillet back on the burner and put it on low heat. If the tomatoes are done cooking, just turn the heat on the burner to low.
Place the tomatoes in with the onions and kale and stir around with a wooden spoon or spatula until they’re evenly spread out in the skillet.
Pour the eggs into the skillet and stir around a bit until everything is evenly distributed.
Using your hand, break off small pieces of goat cheese and drop them into the egg mixture in the skillet, dispersing them as evenly as you can.
Let the egg mixture cook on the stove for 2-3 minutes to help set the bottom and sides.
Place the skillet in the oven and set the time for 20 minutes. Once the timer goes off, use a toothpick to check if it’s done (insert it into the middle of the frittata and if it comes out with any gooey egg bits, it’s done). If it’s not done, put it back in the oven for another 3-4 minutes. Continue checking the frittata until a toothpick inserted in the middle comes out clean.
Once done, take the frittata out of the oven and let rest for 5 minutes to cool slightly. Then cut it into slices with a knife or serving utensil and serve.