Step 1

Heat oil in a large pot over medium heat. Add onion; cook and stir until translucent, about 3 minutes. Add garlic; cook until fragrant, about 1 minute. Add tomatoes with all their juices. Cook and stir until softened, 3 to 5 minutes.

Step 2

Pour wine and clam juice into the pot. Bring to a gentle boil; add mussels. Simmer, covered, until mussels have opened, 4 to 6 minutes. Add parsley. Stir in butter until melted, about 1 minute.