Toss together the ham, onion, cheese and tomatoes in a bowl. Whist the eggs with the milk in another bowl.
Keeping the pastry shells in their metal wrappers, place them on a baking sheet. Place about 1tbsp of the ham and cheese mixture in each of the shells. Gently pour some egg mixture into each, leaving just a bit of space at the top as the filling will puff up.
Bake for 15 minutes, until the egg is just set. Allow to cool before serving. Once they're completely cool you can stack them in a container for packing lunch.