This is just the ticket for spicy-food aficionados. Serve it over rice-and keep a fire extinguisher nearby. If your taste is a little less adventurous, start with only 2 teaspoons Cajun seasoning and 1/2 teaspoon dried crushed red pepper flakes; then adjust as the mixture cooks.
Heat oil in heavy large skillet over medium heat. Add mushrooms, onion, green pepper and garlic and sauté until onion is translucent, about 5 minutes. Add Cajun seasoning and crushed red pepper and stir 1 minute. Mix in crushed tomatoes and stewed tomatoes with their juices; simmer until sauce is thick, stirring occasionally, about 15 minutes. Add shrimp and simmer until just cooked through, about 5 minutes.