In a large pot, melt the butter over medium heat. Add the chopped onion and sauté until translucent, about 3-4 minutes.
Stir in the minced garlic and cook for an additional 1-2 minutes until fragrant.
Pour in the white wine and bring to a simmer. Allow it to cook for about 5 minutes to reduce slightly.
Add the diced tomatoes, oregano, red pepper flakes (if using), salt, and pepper. Stir well and let it simmer for another 5 minutes.
Add the cleaned mussels to the pot, cover with a lid, and cook for 5-7 minutes, or until the mussels have opened. Discard any mussels that do not open.
Remove from heat and stir in the chopped parsley. Serve immediately with lemon wedges on the side.