In a large bowl, whisk eggs, milk, salt, and pepper.
Heat a 12-inch cast iron skillet over medium-high heat. Once hot, melt the butter in the skillet.
Add the chopped shallot and green onion, sauteing for about 3 minutes until fragrant and translucent.
Add the zucchini slices and saute for 1 minute. Move the zucchini around the skillet evenly, then whisk the egg mixture again and pour over the top.
Sprinkle white cheddar and crumbled goat cheese over the top of the frittata.
Bake in the preheated oven for 20-30 minutes or until the center is set. Let cool for 10 minutes before slicing and serving. You can serve frittata warm or at room temperature.
Notes
Use your broiler for a couple of carefully monitored minutes to achieve a golden brown, cheesy top.
Dairy is important for making a fluffy frittata, but too much can cause it to fall apart.
It's a good idea to measure the salt, as the cheese adds a lot of salt, as well, and it can easily become too much.