The fresh mayonnaise: Take a measuring cup and pour in the oil. Break the egg and add it to the oil. Season with pepper and salt.
Add the vinegar, mustard, and lemon juice. Place the hand blender at the bottom and slowly pull it up, mixing until everything is nicely combined. Voila: mayonnaise! Put it in a jar with a lid and place it in the refrigerator.
The mussels: Prepare the mussels in a very large pot, or make them in two batches.
Thoroughly wash the mussels. Sauté the onion, fennel & garlic in a cooking pot with butter. Deglaze with sherry and let simmer for 1 minute.
Add the coarsely chopped chorizo, thyme, and bay leaf and cook for 2 minutes. Add the tomato sauce and bring to a boil. Add the mussels, mix well, and cover with a lid. Shake well and the mussels are done after 4 to 5 minutes.
Serve with (fresh) fries and mayonnaise. That's a celebration!
Tips
Instead of chorizo you can also use cut merguez sausages in pieces, as long as it is spicy.