Preheat the oven to 350°F (180°C) and butter a 9-inch (23 cm) pie plate.
Whisk together the eggs and tomatoes in a large bowl. Add cheese, basil, salt, and pepper; mix well.
Soften zucchini and onion in butter in a non-stick skillet over medium heat. Add spinach and cook briefly, then season with salt and pepper.
Combine the spinach mixture with the egg mixture, pour into the pie plate, and bake for 25 minutes until golden and slightly puffed.
Serve hot or cold.
