Make the Filling:
Heat the olive oil in a large skillet. Add the onion and garlic and stir fry until the onion turns transparent.
Add the spinach and mix thoroughly. Stir-fry, stirring for 2 minutes until the spinach wilts.
Assemble the Tart:
in a separate bowl, whisk together 1 cup of the shredded mozzarella, heavy cream, milk, and eggs. Season with kosher salt and freshly ground pepper to taste.