Preheat the oven to 200 °C.Place baby spinach in a colander and pour over a kettle of boiling water to quickly blanch the spinach. Squeeze out all the excess water and roughly chop. Heat olive oil in a cast iron pan. Fry mushrooms until they release their liquid and begin to brown. Add the baby fennel and half the spring onions. Cook until just tender. Add the spinach to the pan and mix. Season. Whisk together the eggs and milk. Season well with salt and pepper. Turn the heat down to medium low and pour the egg mixture over the vegetables. Distribute things nice and evenly. While the mixture is cooking arrange the smoked salmon and cheese on top of the egg mixture. Once the frittata starts appearing cooked around the edges transfer it to the oven. Bake for 15-20 minutes or until the eggs are set and the frittata has slightly puffed up. Scatter with the remaining spring onions and some fresh dill and serve.