
Preheat your oven to 180°C (fan 160°C, gas mark 4). In a large bowl, whisk the eggs with the milk, salt, and black pepper until well combined and slightly frothy.
Heat the olive oil in a large ovenproof frying pan over medium heat. Add the chopped onion and red pepper, cooking until the onion is translucent and the pepper is soft, about 5-7 minutes.
Add the sliced mushrooms to the pan and cook for an additional 5 minutes until they are browned and tender. Stir in the cherry tomatoes and cook for another 2 minutes until they start to soften.
Stir in the spinach and cook until wilted, about 2 minutes. Pour the egg mixture over the vegetables in the frying pan, ensuring it covers all the ingredients evenly.
Sprinkle the grated cheddar cheese over the top and transfer the pan to the preheated oven. Bake for 20-25 minutes or until the frittata is set and the top is golden brown.
Remove from the oven and let it cool slightly. Garnish with fresh basil before slicing and serving warm. Enjoy your hearty frittata as a standalone meal or with a side of crusty bread.
