In a large Dutch oven or large pot, heat olive oil over medium-high heat. Saute garlic and onions until soft, stirring continuously, approximately 10 minutes.
Add tomato puree through Italian seasoning, stir well. Reduce heat to the lowest setting and simmer for 2 hours, stirring sporadically.
Thirty minutes before serving, stir in sugar. Ten minutes before serving, stir in shrimp and squid. The heat from the sauce will cook seafood.
Serve sauce over pasta of choice and enjoy your Seafood Fra Diavolo!