Boil the carrots in lightly salted water for 4 minutes, then drain. You can pour the hot cooking water over the peas if you like.Leave both the carrots and the peas to drain in a sieve.
Sauce
Melt the butter in a saucepan and whisk in the flour until smooth – let it cook through over a medium heat for a couple of minutes, stirring constantly.
Add the stock a little at a time, whisking well after each addition.
Add the milk in a couple of goes, whisking thoroughly, then let the sauce simmer for about 10 minutes. Whisk regularly so it doesn't catch on the bottom.Season with salt and pepper.
Add the filling – ham, peas and carrots – and heat everything through thoroughly.Meanwhile, warm the tartlet cases in the oven as directed on the packet.
Taste the sauce and adjust the seasoning with salt and pepper before spooning the filling into the tartlet cases, then serve topped with finely chopped parsley.