Preheat the oven to 350°F (170°C).
Bring a large pot of water to the boil and add a pinch of salt.
Meanwhile, heat 2 tablespoons of the olive oil in a large frying pan over medium heat.
Sauté the leeks for 2 to 3 minutes.
Add the remaining 2 tablespoons of oil to the pan along with the red pepper and mushrooms.
Sauté the vegetables for a further 3 or 4 minutes until they have begun to soften.
Meanwhile, cook the pasta in the pot of boiling salted water according to package directions.
When the vegetables in the frying pan have begun to soften, sprinkle the flour on top and stir though.
Cook for a minute or two.
Gradually add the chicken stock, stirring and allowing the sauce to thicken after each addition.
Gradually add the milk, stirring and allowing the sauce to thicken after each addition. After you have added about 1¼ cups milk, stir in the pepper and all but three tablespoons of the Parmesan cheese. Set the remaining cheese aside.
Check the thickness of the sauce. You want a smooth sauce that will cling nicely to the pasta, so if necessary add a bit more milk until this consistency is achieved.
Stir the chicken and ½ cup of the slivered almonds into the sauce.
When the pasta is cooked, drain it and return to the saucepan.
Add the chicken, vegetable and cheese sauce to the pasta and stir gently but thoroughly to coat.
Transfer the mixture to lightly greased 13 x 9 inch casserole dish and sprinkle with the remaining almonds and Parmesan cheese.
Bake in the oven for 25 to 30 minutes until the top of the casserole is beginning to turn golden brown.