Heat up the oil in a large, deep pot over a medium-high heat, then add the chicken, and cook it for 2 minutes.
Reduce the heat slightly to medium, add the sliced onion and sliced red pepper, and sautée for 5 minutes until the onion is translucent and softened.
Add the garlic cloves and the Cajun seasoning, and cook for another 2 minutes.
Toss in the can of chopped tomatoes, chicken stock, penne pasta, and salt and pepper to taste. Cover with a lid, and allow the pasta to cook for 8 minutes, or according to the package instructions.
When the pasta is al dente, add the lemon juice and the whipping cream, and allow everything to cook together for 2 minutes. Serve immediately with a sprinkling of fresh parsley, and optional grated Parmesan.