Creating these Italian Sub Squares is as easy as following these simple steps:

1. Preheat Oven: Preheat your oven to 375°F (190°C). Grease and flour a 9×13-inch baking dish.
2. Mix Dry Ingredients: In a large bowl, combine the flour, baking powder, and salt. Stir well.
3. Combine Wet Ingredients: In another bowl, whisk together the milk, olive oil, and eggs until fully blended.
4. Combine Mixtures: Gradually add the wet mixture to the dry mixture, stirring until smooth.
5. Add Cheese and Meats: Fold in the mozzarella cheese, diced salami, sliced pepperoni, diced bell peppers, and black olives. Mix until all ingredients are evenly incorporated.
6. Season: Add the Italian seasoning and mix to distribute evenly throughout the batter.
7. Pour into Baking Dish: Transfer the mixture into the prepared baking dish, smoothing the top.
8. Bake: Place in the preheated oven and bake for 25-30 minutes, or until golden and a toothpick inserted into the center comes out clean.
9. Cool: Remove from oven and let cool in the pan for about 10-15 minutes.
10. Slice: Cut into squares and place on a serving platter.
11. Garnish: Top with shredded lettuce, pepperoncini, and fresh basil if desired.

These steps will guide you flawlessly in creating this Italian delight!