If using beef fat:

Chop the fat into small pieces, about the size of your fingertip. 

Add it to a deep skillet with 1/2 cup (125ml) water. Bring the water to a simmer over medium heat. 

Once the water evaporates, reduce heat to low. 

Let cook slowly for 3-5 hours, stirring every once in a while. The fat will render into liquid, leaving little crunchy bits behind. 

Strain the fat through a mesh strainer into whatever you plan on frying the potatoes in. Make sure there is lots of headspace for expansion. 

Cut the potatoes into french fries, put them in a bowl, and cover with cold water. 

Let sit for as long as possible, up to overnight. I like to do it while the beef fat is rendering. 

Heat the fat or oil up to 300°F. 

Drain the potatoes and let dry for a bit. 

Add them to the oil and cook for 5 minutes. 

Take the fries out of the oil and drain. 

Increase the heat of the oil to 375°F.

Carefully dump the fries back in and cook until golden and crispy. 

Remove the fries and drain. Season with fine salt. 

Wash the mussels under cold water, pulling off any beards (tough seaweed-looking bits coming out of the shell that the mussels use to attach themselves to rocks), and discarding any open mussels that don’t close after a couple gentle taps on the counter. 

Cut the dark green section off of the leeks and discard (or wash and save for making broth). Cut the white parts in half lengthwise and run them under cold water, leafing through the layers to find any potential dirt that may be trapped. 

Slice them into thick half moons. 

Melt the butter in a large pot over medium heat. 

Add the leeks, shallots, and celery and cook 5 minutes. 

Add the garlic, bay leaves, thyme, and pepper and cook for another 2 minutes. 

Add the white wine, bring to a simmer, then dump in the mussels, tossing them with the vegetables and wine. 

Put a lid on the pot and cook for 5-8 minutes, or until the mussel shells open up. 

Take off of the heat once they open, or they will overcook and shrivel. 

Toss the mussels in the liquid at the bottom of the pot a few times, then scoop into bowls. 

Add a handful of fries to each bowl, as well as a dollop of mayo if desired. 

Use a mussel shell or fork to pry the mussels out of their shells and toss the spent shells into a large communal bowl. Discard any mussels that didn’t open.

I like to let some of the fries soak up the liquid and some to stay crunchy. Enjoy!