Preheat the oven to 180 ºC.
In a large bowl mix the eggs, milk, cheese and chives. Season with salt and black pepper and whisk well to combine. Set aside.
Place a large non-stick, ovenproof frying pan over medium heat and add the olive oil. When the oil is ready, add the onions and fry for a few minutes until translucent. Add the garlic and chilli and fry for a minute until fragrant. Add the spinach and saute until it wilts, then remove the pan from the heat and set aside.
Place a separate pan over medium heat, add the haddock pieces and fry for 2 minutes. Be careful not to break the haddock up too much. Remove from the heat and drain any liquid released from the fish.
Turn the heat down to medium low and place the pan with the fried spinach and onion back over the heat; spread the spinach and onions out evenly in the pan. Pour the egg mixture in evenly to cover all of the ingredients. Add the pre-cooked haddock pieces evenly around the pan.
Cook stovetop on low for 4-5 minutes until the egg mixture sets around the edges of the pan, then transfer to the middle rack of the oven with the grill on.
Grill the frittata for 5 minutes until it is set. Remove from the oven and allow to cool slightly before slicing into wedges and serving.