Preheat the oven to 350°F (175°C). Grease a 9x13-inch baking dish or line it with parchment paper.
In a large bowl, combine the grated zucchini, flour, baking powder, salt, and black pepper. Mix well to combine.
In another bowl, whisk together the eggs, milk, and olive oil until smooth.
Pour the egg mixture into the zucchini mixture and stir until just combined.
Fold in the shredded cheddar cheese, grated Parmesan cheese, chopped green onions, and diced bell pepper.
Spread the mixture evenly in the prepared baking dish.
Bake for 30-35 minutes, or until the top is golden brown and a toothpick inserted in the center comes out clean.
Allow to cool for 10 minutes before slicing into squares.