Cut the chicken breasts into bite-size pieces and season with half of the salt and freshly ground black pepper.
Heat the olive oil in a large skillet over medium-high heat.
Add the chicken and sear until golden brown on all sides, approximately 5-6 minutes. Once done, remove the chicken from the skillet and set aside.
Lower the heat to medium and add the butter along with the chopped garlic to the skillet. Once the butter has melted, sprinkle the flour over the butter and whisk it in, cooking for about 1-2 minutes until you have a light golden roux (it's the mixture of cooked butter and flour that will help thicken your sauce).
If you're using tomato paste, mix it in now and cook for another minute.
Gradually pour in the chicken stock, whisking constantly to incorporate the roux and avoid any lumps. Allow the sauce to simmer for a few minutes, stirring occasionally, until it starts to thicken.
Stir in the double cream (heavy cream), and mix well.
Add the chopped sun-dried tomatoes, paprika, chili flakes, grated parmesan, the rest of the seasoning, and dried oregano. Let the sauce simmer for a couple more minutes so the flavors meld.
Return the chicken pieces to the skillet along with any resting juices. Cover the pan, and cook over medium heat for 5-6 minutes, or until the chicken is cooked through (165°F / 74°C) and the sauce has thickened to your liking.
Meanwhile, cook the penne in a large pan of generously salted boiling water until al dente, following the packet timings. Reserve a cup of the pasta water, then drain.
Add the drained penne to the skillet and toss until every piece is coated in the sauce. If it looks too thick, loosen it with a splash of the reserved pasta water.
Take the skillet off the heat. Adjust the seasoning if needed, squeeze in the lemon juice, and garnish with fresh basil leaves.