In a large pot, add the olive oil. Once warm, add the butter and stir gently till it's melted.
Add the garlic stir for about 1-2 minutes before adding the diced onions, and then stir for another 1-2 minutes.
Next, add the diced capsicums and stir well. Allow these vegetables to cook for about 5-7 minutes. Add a pinch of salt but not too much at this point.
Once the vegetables are soft, add the mussels and water and bring to a simmer. Put on the lid and allow mussels to simmer over low heat.
Add the cream and stir well while bring to boil. At this point, add a little more salt and pepper to taste.
Serve immediately and enjoy!