In a bowl, whisk together the eggs, milk, salt, and black pepper.
Heat the olive oil in a non-stick skillet over medium heat. Add the pork, onion, bell pepper, and zucchini. Cook for 5-7 minutes, until the vegetables are tender.
Add the baby spinach to the skillet and cook for an additional 1-2 minutes, until wilted.
Pour the egg mixture over the vegetables in the skillet. Cook for 2-3 minutes, until the edges start to set.
Sprinkle the shredded cheese over the top of the frittata. Transfer the skillet to the oven and broil for 3-5 minutes, until the cheese is melted and golden brown.
Remove the skillet from the oven and let the frittata cool for a few minutes before slicing and serving.