3. Add the spinach to the mixture (you may have to add half at a time) and cook until wilted and soft. Drain the vegetable mixture well in a large colander to remove any excess liquid.
4. In a big bowl, beat the eggs, milk and crumbled feta together, then add the vegetable mixture and mix to combine. Pour into the prepared baking dish and sprinkle the top with gruyere.
5. Bake in the preheated oven for 35-40 minutes until golden brown on top. To check if the frittata is ready you can do the toothpick test - insert a toothpick in the centre and if it comes out clean it is done!
Cool for 15 minutes before cutting. Serve hot, at room temperature or chilled, with a side salad, spicy home-made salsa or fruit.