Heat 1 TBSP of olive oil in a pan. Add the garlic and onions, and simmer for about 2-3 minutes. Add the diced tomatoes, grape tomatoes, and the bell pepper, and season with the rest of the Cajun seasoning and the red pepper flakes. Give everything a good stir, and let it all simmer for about 10-15 minutes. The grape tomatoes will soften up and burst.
While the sauce is simmering, cook your pasta, drain it, and transfer it back to the pot.
Sprinkle the chopped fresh basil into the pasta sauce, stir it, then transfer the sauce into the pot over the pasta, stirring to mix it so that the pasta is evenly coated; set aside.