In a large stock pot or Dutch oven, heat the olive oil over medium-high heat. Add the onion, salt, and pepper, and cook until soft but not browned, about 5 minutes. Add the garlic, red pepper flakes, Italian seasoning, and tomato paste. Cook, stirring, for another minute.
Add the ground peeled tomatoes and then fill the can halfway up with water and add that along with the wine and sugar. Bring to a boil, cover, reduce heat and simmer for 30-60 minutes, stirring occasionally. (The sauce can be made ahead to this point, store covered in the refrigerator for up to several days.)
Add the mussels and cook, covered for 4-5 minutes or until the mussels have opened. Discard any mussels that do not open.
Using a slotted spoon, divide the mussels between 4 soup bowls then ladle in the sauce. Garnish with the parsley and serve.