
Preheat oven to 375°F (190°C). Lightly grease a 9-inch pie dish or baking dish.
Arrange tomato slices on a paper towel-lined tray, sprinkle lightly with ½ teaspoon salt, and let sit for 30 minutes to release excess moisture.
Pat dry the tomatoes to remove as much moisture as possible. Set aside.
In a large bowl, combine cheddar, mozzarella, Parmesan cheese, eggs, remaining salt, pepper, and garlic powder.
Spread about ⅓ of the cheese mixture on the bottom of the prepared dish. Then layer half the tomato slices evenly in the prepared dish.
Spread another ⅓ of the cheese mixture on top of the tomatoes and sprinkle with basil and green onions.
Layer the second half of the tomatoes on top and spread with the remaining cheese mixture.
Bake for 35–40 minutes until the center is set and the top is lightly golden. Let cool for 10–15 minutes before slicing and serving.