Preheat the oven to 180°C conventional oven/ 160°C fan-forced (356° Fahrenheit/Gas 4). Grease a 24cm round baking dish with butter.
Thinly slice one of the zucchini to a thickness of approximately 2mm. Chop the remaining zucchini into 1cm cubes.
Heat the extra virgin olive oil in a medium-sized frying pan over medium heat. Add the garlic clove, cubed zucchini and sweet potato. Season with sea salt. Cook, stirring occasionally, for 6 minutes to colour slightly. Stir in the freshly chopped mint. Transfer the cooked zucchini and sweet potato to evenly cover the butter greased baking dish. Discard garlic clove.
Place the zucchini rounds in a single layer into the frying pan. You may need to cook in 2 batches. Cook the zucchini rounds for 2-3 minutes on each side to lightly brown.
Crack the eggs in a bowl, then add the Parmigiano Reggiano and mint. Season well with sea salt and use a fork to lightly whisk until combined.
Pour the egg mixture over the vegetables. Arrange the zucchini rounds over the egg mixture. Bake the frittata for 25 minutes.
Allow the Baked zucchini frittata to cool for 5 minutes before serving – buon appetito!