Rinse the mussels under cold water, discard any with broken shells, and debeard them by pulling off the fibrous beard. Set aside in the refrigerator while you prepare the other ingredients.

Sweat aromatics

In a large wide pot or deep sauté pan with a tight-fitting lid, heat the olive oil and butter over medium heat until butter foams. Add the chopped onion and a pinch of salt and sweat until translucent, about 4–5 minutes. Add the sliced garlic and sliced red chili and cook 1 minute more until fragrant, taking care not to brown the garlic.

Deglaze with fino sherry

Increase the heat to medium-high, pour in the fino sherry and scrape any browned bits from the bottom of the pan. Let the sherry boil for about 1 minute to burn off most of the alcohol and concentrate flavor.

Add stock and simmer

Add the chicken or shellfish stock and bring the mixture to a simmer. Season with the measured sea salt and black pepper. Taste the broth and adjust seasoning if needed.

Cook mussels

Add the cleaned mussels to the simmering broth, cover with the lid, and steam for 4–6 minutes, shaking the pan once or twice. Check after 4 minutes; mussels are done when most have opened. Use tongs to remove any remaining closed mussels and discard if they do not open after a minute more.

Finish broth

Remove the pan from heat. Stir in the fresh lemon juice and chopped parsley. Taste and adjust seasoning with additional salt, pepper, or a splash more sherry or lemon if desired.

Petite Frites (small fries)

Prepare potatoes

Cut the potatoes into very thin sticks about 1/4" wide (petite matchsticks). Rinse the cut potatoes in cold water until the water runs clear to remove excess starch. Pat completely dry with clean kitchen towels or paper towels to avoid oil splatter.

Heat oil and fry (first fry)

Pour oil into a heavy-bottomed pot or deep fryer and heat to 300°F (150°C). Fry the cut potatoes in batches for 3–4 minutes until softened and pale (they should not be brown). Remove with a slotted spoon and drain on paper towels.

Finish fries (second fry)

Increase the oil temperature to 375°F (190°C). Fry the par-cooked potatoes in batches for a second time until golden and crisp, about 2–3 minutes. Drain on paper towels, immediately season with fine sea salt, flaky sea salt to taste, and the measured black pepper. Toss with optional chopped parsley if using.

Plating & Serving

Divide the petite frites between two shallow bowls or plates (or serve on the side). Spoon the mussels and a generous ladle of the sherry-chili broth into a wide shallow serving bowl so mussels sit in the broth.

Garnish the mussels with a little extra chopped parsley and serve with lemon wedges on the side for squeezing. Serve immediately while mussels are hot and fries are crisp.

Optional: Provide crusty bread for soaking up the broth and small forks for the mussels; serve fries alongside or in a paper cone for a bistro feel.