Place Butter (30 milliliter) in the casserole dish over medium heat and sauté the Leek (4) until soft. Increase the heat, add the Parsnip (3) and cook to al dente.
Strip the Fresh Thyme (2 sprig) from the woody stalks and add to the dish. Simmer the mixture for another minute then deglaze the dish with the Dry White Wine (100 milliliter). Reduce by half.
Once reduced, add the Fresh Cream (250 milliliter), Chicken Stock (50 milliliter), and the Garlic (4 clove). Scatter in the Grated Parmesan Cheese (100 gram) and stir to combine. Season with Coarse Sea Salt and Pepper (to taste), going easy on the salt.
Once the sauce has thickened, add the Fresh Mussels (3 kilogram). Turn the heat to medium and place all the mussels into the sauce. Cover the dish with its lid and steam for about 6 to 8 minutes, or until all the mussels have opened
Discard any that haven’t opened unless you’re sure of the origin and freshness of the mussels. If they’re very fresh and have opened a crack, simply work the shell wider with the tip of a butter knife.
Stir the mussels through the sauce and divide between four warmed bowls. Serve the mussels immediately with lemon wedges to squeeze over the top, fresh crusty bread to mop up the sauce and a chilled bottle of Vriesenhof wooded Chardonnay.