- With the rack in the middle position, preheat the oven to 350°F (180°C). Butter a 9-inch (23 cm) pie plate.
- In a large bowl, whisk together the eggs and tomatoes. Add the cheese and basil. Season with salt and pepper and mix well. Set aside.
- In a non-stick skillet over medium heat, soften the zucchini and onion in the butter. Add the spinach and cook for 1 minute. Season with salt and pepper.
- Add the spinach mixture to the bowl with the egg mixture. Combine well and pour into the pie plate. Bake for 25 minutes or until the frittata is golden and slightly puffed. Serve hot or cold.