Heat olive oil in a pan, add your chopped onion and garlic. Gently fry for 2 minutes.
Add Knorr Tomato Base dry Cook-in-sauce, chopped tomatoes and fish stock, stir and leave to simmer for 5 minutes.
Season with salt and pepper to taste
Add mussels, cover with lid and leave to simmer until all mussels have opened, about 3 minutes. (Remove any mussels that haven’t opened)
Remove from heat, and serve with toasted bread.