Rinse mussels under cold water. Discard any that are cracked or remain open when tapped. Use a small knife to remove beards (fibrous threads) by pulling toward the hinge. Set cleaned mussels in a bowl in the refrigerator while you prepare the sauce.

Sauce & Cooking

Sweat aromatics

Heat a large wide pot or deep skillet over medium heat. Add olive oil and 1 tablespoon butter. When the butter is melted and foaming, add the chopped onion and a pinch of salt. Cook, stirring occasionally, until the onion is translucent and softened, about 4–5 minutes. Add the minced garlic and red pepper flakes (if using) and cook for 30–45 seconds until fragrant, taking care not to brown the garlic.

Add tomatoes and deglaze

Add the diced tomatoes to the pot and cook 2–3 minutes to break them down slightly. Pour in the white wine and use a wooden spoon to scrape any browned bits from the bottom of the pan. Increase heat to medium-high and bring the wine to a gentle simmer for 2 minutes to evaporate some alcohol and concentrate flavor.

Add mussels and steam

Add the cleaned mussels to the pot in an even layer. Sprinkle the chopped tarragon, remaining 1 tablespoon butter, lemon juice, salt, and black pepper over the mussels. Cover the pot with a tight-fitting lid and let steam for 4–6 minutes, shaking the pot gently once halfway through, until the mussels have opened. Check for doneness: most mussels will open at 4–6 minutes; discard any that remain closed after cooking.

Finish the sauce

Remove the pot from heat. Using a slotted spoon, transfer the mussels to a serving bowl, leaving the cooking liquid in the pot. If you prefer a slightly thicker sauce, simmer the liquid uncovered over medium heat for 1–2 minutes. Stir in chopped parsley and adjust seasoning with additional salt, pepper, or lemon juice to taste. Spoon sauce over the mussels.

Serve

Serve the mussels immediately with crusty bread to soak up the white wine-tomato sauce. Garnish with extra tarragon or parsley if desired.