1/2 pound fettuccine 
4 tablespoons oil, preferably olive, divided 
1 teaspoon crushed red pepper flakes, divided 
1 pound shrimp, peeled and devained 
2 onions, finely chopped (about 1 cup) 
3 cloves garlic, minced 
1 cup marinara sauce from a jar 
1 tablespoon chopped fresh parsley 

Cook pasta according to package directions; drain. Meanwhile, in large skillet heat 2 tablespoons oil over medium-high heat. Add 1/2 teaspoon pepper flakes; cook, stirring constantly, 30 seconds. Add shrimp; cook, stirring, until pink, about 5 minutes. Remove; set aside. In same skillet heat remaining oil over medium-high heat. Add onions, garlic and remaining pepper flakes. Cook until onions are slightly browned, 5-7 minutes. Stir in marinara sauce; heat through, 1-2 minutes. Remove from heat. In large bowl combine marinara mixture with reserved pasta and shrimp. Serve sprinkled with parsley. 

Ready to serve in 25 minutes.