Preheat the oven to 190ºC/Fan 170ºC/Gas 5. Melt the butter in a small frying pan and add the leek and garlic. Cook over a medium heat for 8–10 minutes, stirring occasionally, until softened. Remove from the heat and set aside to cool.
Beat the cream and egg together in a medium bowl, then fold in the leek mixture along with the salmon and dill. Divide between the tartlet cases.
Sit the tartlets on 2 large baking sheets and bake for 10–15 minutes, or until the filling is just set. Garnish with dill fronds and serve. You can change the flavour of these tartlets by swapping the dill for chopped fresh chives, or make a selection of both to serve at your party.