1. In a large pot, combine beer and mussels, cover and steam until just opened, about 2-3 minutes.
2. Using a slotted spoon, remove steamed mussels from the pot and set aside.
3. Strain the cooking liquid through a small strainer lined with several layers of cheesecloth. Reserve and set aside.
4. Remove mussels from shells, toss with olive oil, lemon, herbs to make a salad. Season with salt and pepper.
1. Melt butter in a large pot over medium heat. Add leek whites, onions, garlic and cook until translucent but not brown.
2. Add potatoes, cream, milk, and mussel-beer cooking liquid. Bring to a boil and cook until soft. Season with salt. Blend until smooth in a high speed blender.
3. Add about 1 quart (1 L) to a whipping siphon such as an ISI gun. Charge with 2 N2O chargers and wait 5 minutes, then add 1 more N2O charger. Shake well, keep warm. Reserve remaining soup for another use.
1. Peel a large russet potato and cut a flat side. Place the flat side down and slice on the ruffle blade of a mandoline, then turn 90 degrees and slice again to create a waffle pattern. Repeat process to create several fries.
2. Use a 2 inch (5 cm) cookie cutter to trim the waffled potatoes into even rounds (or cut by hand).
3. Heat 1-2 inches of oil in a large heavy pot to 325 F (160 C). Fry the prepared potatoes for 4-5 minutes, flipping once. Transfer to a lined plate to drain, season with salt.
1. On a dark plate, make 4 rounds of the mousse in different sizes, following the right curve of the plate, 1-2 inches (2.5-5 cm) from the side.
2. Place 3 spoonfuls of chilled mussel salad between the rounds of mousse.
3. Lean waffle fries around the mussels and mousse to finish.