Trim both ends of the leek and wash off any dirt, then finely slice the leek and set it aside. Peel mushrooms and slice. If you have large mushrooms, then also slice in half. Peel the cloves of garlic and cut of a tiny part of the ends and any bruising. Then slice and coarsely chop the garlic.
Boil water in a pot and add a dash of olive oil and salt. Then add the penne and let that cook. Occasionally stir the pasta and check to see if it is ready while you cook the sauce.
Heat a pan and add the butter. Once it has melted, add the garlic and let that become golden. Now you can add the leek. Cook the leek until is has become relatively wilted and then gradually add the cream. Let the pan heat the cream and then add the spices. Add your mushrooms now, and don't worry if they break up a bit while you're stirring. Stir in the cornflour and let the sauce simmer, occasionally stirring it. (Simmering should take about 10 - 15 mins on a medium to low heat)
When your pasta is ready, drain it and add another dash of olive oil and stir that through. Serve pasta on a plate or a bowl and pour the sauce on top, stir it through if you like.