In a large pan (ideally cast-iron) set oven medium-low heat, sweat out the leeks and garlic in butter, stirring for 5 minutes. Add the spinach and stir again.
Combine the eggs and milk, add to the pan, and stir. Add the remaining ingredients, season, and stir to combine. Bake in the centre of the oven for 18-20 minutes, or until frittata is slightly golden. Garnish with fresh dill when serving, if desired.