Preheat your oven to 200°C (180°C fan).
Heat half the olive oil in a large, ovenproof frying pan over medium heat. Add the sliced onion, capsicums, and a pinch of salt. Cook, stirring occasionally, until the vegetables are soft and just starting to caramelise (about 8–10 minutes).
Add the garlic and harissa, cooking for a further 2 minutes until fragrant. Stir in the chopped spinach, allowing it to wilt slightly. Remove from heat.
In a large bowl, whisk together the eggs, cream, a generous pinch of salt, and a good crack of black pepper. Whisk until voluminous (you can use an electric mixer if you prefer).
Pour the egg mixture over the sautéed vegetables, gently stirring to ensure the fillings are evenly distributed. Scatter the crumbled feta over the top.
Transfer the pan to the preheated oven and bake for 30 minutes, or until the frittata is just set and lightly golden on top.
Allow to cool slightly before slicing into small squares. Scatter with fresh parsley and serve with lemon wedges and a big green salad.