Fill a large pot with water and set it over high heat to boil.

While the water heats, open the tins of smoked mussels. Drain the mussels into a small bowl, reserving all of the olive oil from the tins in a cup or small bowl.

Finely chop the shallot, thinly slice the garlic, zest and juice the lemon, and chop the parsley.

When the water comes to a full boil, add enough kosher salt so the water tastes lightly seasoned.

Add the spaghetti to the boiling water in the pot. Stir for the first 30 seconds so the strands do not stick together.

Cook the spaghetti until it is tender but still a little firm in the center, using the package time as a guide.

While the pasta cooks, set a large skillet over medium heat.

Add the reserved mussel oil and the butter to the skillet. Let the butter melt completely.

Add the chopped shallot to the skillet. Cook for 3 to 4 minutes, stirring often, until the shallot is soft and lightly golden at the edges.

Add the sliced garlic and red pepper flakes to the skillet. Cook for about 30 seconds, stirring, until the garlic smells fragrant but has not browned.

Pour the heavy cream into the skillet. Add the lemon zest and stir to combine.

Bring the sauce to a gentle simmer and let it bubble for 2 minutes, until it thickens slightly but is still loose.