Melt the butter in a sauté pan over medium heat and cook the onions for 4 minutes until tender.
Add the garlic and cook an additional minute then set aside to cool.
Whisk eggs in a bowl and stir in ricotta, mozzarella and parmesan cheeses.
Once combined, stir in drained spinach, salt and cooled onion and garlic mixture.
Pour the mixture into greased mini muffin cups.
Bake for 20-25 minutes or until the filling is set and golden on top. If necessary use a sharp knife to cut around sides of bites to release.
Notes
To Store: These bites can be stored in an airtight container in the fridge for up to 3 days. To Freeze: Place the cooled egg bites on a baking sheet in a single layer and freeze until firm. Transfer them to a freezer-safe bag or container, making sure to remove any excess air before sealing. They can be stored in the freezer for up to 3 months. To reheat, simply thaw them overnight in the refrigerator or microwave them on defrost straight from the freezer until heated through.