Rinse mussels under cold water; discard any with broken shells or that do not close when tapped. Using a small knife or your fingers, remove the beard (byssal threads) from each mussel and scrape off any barnacles. Place cleaned mussels in a bowl in the refrigerator until ready to cook.

Sweat aromatics

Heat a wide, heavy-bottomed skillet or sauté pan with a tight-fitting lid over medium heat. Add 2 tbs olive oil. When shimmering, add chopped onion and sliced fennel. Season with a pinch of salt. Cook, stirring occasionally, until softened and translucent, about 6–8 minutes. Add minced garlic and red pepper flakes and cook 30–45 seconds until fragrant, taking care not to burn the garlic.

Build tomato base

Stir in the tomato paste and cook 1–2 minutes to caramelize slightly. Add the crushed tomatoes, scrape up any browned bits from the pan, and stir to combine. Bring mixture to a gentle simmer.

Deglaze and add liquor

Pour in the white wine and Sambuca. Increase heat to medium-high briefly to bring to a simmer so alcohol can evaporate for about 1–2 minutes; this will lift flavor from the pan. Stir in the stock, lemon zest and lemon juice. Taste and adjust seasoning with about 1/2 tsp salt and 1/4 tsp black pepper (adjust to taste).

Cook mussels

Turn heat to medium, add the cleaned mussels to the simmering tomato broth, and immediately cover with the lid. Allow mussels to steam for 5–7 minutes, shaking the pan once or twice, until most mussels have opened. Use tongs to remove any mussels that open early to prevent overcooking. After 7 minutes, check: discard any mussels that remain tightly closed.

Finish and garnish

Once mussels are opened and cooked through, remove pan from heat. Stir in chopped parsley and chopped fennel fronds. Taste the broth and adjust salt, pepper, or lemon juice as desired. Spoon mussels and broth into a warmed shallow serving bowl, reserving some broth for dipping the bread.

Grilled Bread

While the mussels cook (or just before serving), preheat a grill pan or outdoor grill to medium-high, or preheat your broiler. Brush both sides of the bread slices lightly with olive oil and sprinkle one side with coarse sea salt. Grill or broil bread 1–2 minutes per side until nicely charred and crisp but still slightly chewy inside.

Remove grilled bread and immediately rub the toasted surface with the cut side of the garlic clove for a subtle garlic aroma. Set bread on a plate and keep warm under foil or return briefly to the pan to reheat if needed.

To serve, arrange grilled bread around the bowl of mussels. Use the bread to soak up the tomato-fennel broth. Optionally spoon a bit of broth over the bread and top with extra chopped parsley or fennel fronds.