Heat the olive oil in a large stockpot set over medium-low heat. Add the garlic and leek, and sweat until softened, approximately 3 minutes.
Place the mussels in a steamer insert or metal colander and rinse with cold water to remove any excess dirt or grit. Remove any beards with a pair of needle-nose pliers (grab low and twist to remove).
Add the tomato and wine to the stockpot and boost the heat to medium-high to simmer. Add the steamer insert or colander containing the mussels to the pot and cover. Cook for 3 minutes, then check to see if the mussels have opened. If some are still closed, replace the lid and cook for another 30 seconds to 1 minute. Discard any unopened mussels at this point — whatever’s in them, you don’t want. Remove the pot from the heat.
Place the mussels in a serving bowl, reserving 8 proud specimens. Remove these from their shells and add them to the stockpot.
Tipping the pot so that the liquid pools on one side, use an immersion blender to puree the mixture until it reaches a saucy consistency. Pour the sauce over the mussels and garnish with parsley. Serve with a loaf of crusty bread and chilled white wine.