Heat ¼ cup olive oil in a saucepot over medium-low heat. Add the sliced garlic and cook until golden brown, about 3-4 minutes, stirring frequently.
Add the tomatoes, sherry, oregano, and red chili flakes to the same pot. Bring to a boil, cover and reduce heat to low. Continue to simmer for 30 minutes, stirring occasionally. Season the sauce with salt and pepper.
Add the mussels to the sauce, cover and cook for about 7-8 minutes or until the shells open. Remove the mussels and place in a bowl.
Add the butter and fresh basil to the tomato sauce, stirring until the butter has melted. Adjust the seasoning once again with salt and pepper. Ladle the sauce on the mussels and sprinkle grated parmesan on top. Serve with crusty bread.