Step 1

Combine cilantro stems, half of garlic, half of shallot, coriander seed, dried chili, lime zest and a small pinch of kosher salt in a mortar and pestle. Pound and grind until a smooth paste is formed.

Step 2

Scoop 2 tablespoons of thick cream from the top of the can of coconut milk and add to a large saucepan. Add oil and heat over medium heat until shimmering and sputtering. Add remaining garlic and shallots, pounded paste, and the store-bought curry paste. Cook, stirring and scraping the paste around the bottom of the pan until very aromatic, about 4 minutes. Add remaining contents of coconut milk can, sugar, and fish sauce. Bring to a simmer and cook until rich and aromatic, about 3 minutes. Season to taste with more fish sauce as desired.

Step 3

Add mussels, stir, cover, and cook, shaking pan constantly and peeking every 30 seconds to stir. As soon as all the mussels are open, stir in chopped cilantro leaves, fresh chili, and lime juice. Serve immediately, with additional limes at the table.