Rinse mussels under cold running water. Discard any that are cracked or open and do not close when tapped. Use a small brush or paring knife to scrub shells and pull off beards. Transfer cleaned mussels to a bowl and keep refrigerated until ready to cook.

Build the cooking liquid

In a large wide pot or Dutch oven over medium heat, melt 2 tablespoons unsalted butter with 1 tablespoon olive oil. Add the diced shallots and cook, stirring, until softened and translucent, about 3–4 minutes.

Add aromatics

Add the minced garlic, crushed red pepper flakes, and lemon zest to the pot. Cook for 30–45 seconds, stirring constantly, until fragrant but not browned.

Deglaze and simmer

Pour in the cider and white wine (if using) and bring to a simmer, scraping up any browned bits from the bottom of the pot. Add the chicken/seafood stock, thyme sprigs, and bay leaf. Increase heat to medium-high and simmer for 3 minutes to let flavors meld and the alcohol to reduce slightly.

Steam the mussels

Add the cleaned mussels to the pot, cover with a tight-fitting lid, and steam for 4–6 minutes, shaking the pot once or twice, until most mussels have opened. Check for doneness: discard any mussels that remain tightly closed.

Finish the broth

Remove the pot from heat. Using tongs or a slotted spoon, transfer mussels to a large serving bowl and tent with foil to keep warm. Strain the cooking liquid through a fine mesh sieve into a clean saucepan, pressing gently to extract liquid but leaving any grit behind. Return strained liquid to low heat and whisk in the remaining 1 tablespoon unsalted butter, lemon juice, salt, and pepper. Taste and adjust seasoning.

Herbs

Stir the chopped parsley and chives into the broth to wilt slightly and add fresh herbal brightness.

Aromatics & Flavoring

The shallots, garlic, lemon zest, crushed red pepper and lemon juice are incorporated during steps in the Mussels & Broth section. Ensure the garlic is added only after the shallots soften to avoid burning, and add lemon juice at the end to preserve brightness.

To Serve

Toast sourdough

While the mussels are steaming, heat a skillet over medium heat (or use a broiler). Lightly butter sourdough slices and toast until golden and crisp on both sides, about 2–3 minutes per side in a skillet or 1–2 minutes under a broiler. Sprinkle with coarse finishing salt if desired.

Serve

Arrange toasted sourdough slices around the serving bowl. Pour the finished hot broth over the mussels or serve the broth on the side for dipping. Garnish with any remaining chopped parsley and chives. Serve immediately so guests can dip bread into the broth and enjoy the mussels while hot.