For the chips:
Cut the potatoes into even sticks. Preheat oven to 200ºC/390ºF.
Line a baking pan with parchment paper and toss in the chips in a single layer, coating with oil and salt.
Bake for about 45 minutes in the middle of the oven, tossing around once or twice.
Remove from the oven when brown and crispy.
For the mussels:
Clean the mussels with abundant water by scraping their shells, making sure they are closed. Discard any mussels that are open or that do not withstand the pressure of your fingers.
In a large pot fry the garlic cloves in the olive oil on a fairly low hear for 2 minutes. Remove the garlic cloves.
Add the rosemary, wine and water and bring to a boil. Add half of the parsley and salt.
As soon as the pan is hot and filled with steam, add the mussels. Mix them well with the broth and cover quickly.
Cook for 4 minutes until all the mussels have opened (discard those that remain closed).
Sprinkle with the rest of the parsley and serve immediately.