In a pot over medium-low heat, melt butter. Add onion and cook until fragrant and soft, 5 minutes, then add garlic and cook until fragrant, 1 minute more.

Add diced tomatoes, wine, and parsley and stir until combined. Season with salt and pepper.

Add mussels and simmer until all shells are open. (Discard any shells that aren’t open.)

Garnish with more parsley and serve with grilled bread