Set a medium Dutch oven over medium heat and melt the butter.
Add garlic, lemon zest, and shallots, then reduce the heat to medium-low. Sauté until soft and translucent, about 8–10 minutes. Avoid browning. Season with a pinch of salt and pepper.
Whisk in the flour until smooth and cook for 1–2 minutes until it forms a golden paste and loses its raw smell.
Add clam juice and bring to a boil, whisking to fully incorporate the flour. Scrape up any browned bits from the bottom.
Reduce to a simmer and stir in the cream. Season again with salt and pepper.
Add mussels and half the parsley or scallions. Stir and cover with a lid. Steam until mussels open, about 5 minutes. Transfer opened mussels to a bowl with a slotted spoon. Discard any that don’t open after 10 minutes.
Taste the broth. Adjust with salt, pepper, or additional lemon juice as needed. Simmer a few more minutes to thicken, if desired.
Remove from heat, stir in the remaining lemon juice to taste, and return mussels to the pot or transfer broth to the serving bowl.
Garnish with remaining parsley or scallions. Serve immediately with lemon wedges and crusty bread.