Rinse the mussels under cold water and remove any beards.
In a large pot, heat olive oil over medium heat.
Add chopped onions and sauté until translucent.
Stir in minced garlic and cook for an additional minute.
Pour in the white wine and bring to a simmer.
Add diced tomatoes and season with salt, pepper, and red pepper flakes.
Introduce the mussels to the pot and cover with a lid.
Cook for about 5-7 minutes until the mussels open up.
Discard any mussels that do not open.
Garnish with fresh parsley and serve with lemon wedges.