Heat the olive oil in a large pan over medium heat.
Add the chopped onion and the garlic.
Sauté until softened, about 3-4 minutes. Let the garlic become fragrant.
Add the red pepper, stirring occasionally until the pepper softens.
Stir in the dried oregano, salt, black pepper, and a pinch of chili flakes
.Cook for another minute to release the flavors.
Add the tomato paste and stir well, allowing it to caramelize slightly for richer depth.
Add the mussels to the pan and gently stir to coat them in the sauce
.Cover and let them cook for about 10 minutes, or until they are fully heated through.
Pour in the canned tomatoes and fresh diced tomatoes.
If using, add the tsikoudia or ouzo at this stage.
Stir well and let the sauce simmer for about 15 minutes, allowing it to thicken and develop its flavors.
Lower the flame and immediately crumble the feta cheese over the dish, allowing it to slightly melt into the sauce.
Remove from the heat.
Sprinkle with freshly chopped parsley and serve hot with crusty bread for dipping.