Heat large pot with olive oil and sliced garlic and onions.
When aroma is released, add mussels.
Add lemon juice, herbs and clam juice (optional), and gently toss.
Add tomatoes. Cover and simmer over medium heat until mussels are steamed open, generally 3 to 6 minutes.
Remove pot from heat. Discard unopened mussels.
To finish mussels, add butter and swirl to make broth thicker.
Optional: Serve with crusty bread for dipping in broth.