In a large pot, heat wine over medium heat. Add the mussels to the pot, cover, and cook until they open, about 5–7 minutes. Discard any mussels that remain closed. Remove the mussels from the broth and reserve.

Repeat with the clams. Deshell all but a few of each, reserving the meats.

To the clam-mussel broth, add the butternut and onion trim, kelp, and 8 cups of water. Simmer for 10 minutes over medium heat, then strain, reserving the liquid and discarding the solids.

In a large, heavy-bottomed pot over medium heat, add the butter and onion and cook, stirring occasionally, until softened and caramelized, 20–30 minutes.

Add the butternut squash and coriander. Cook for another 5 minutes, stirring occasionally, until it begins to soften.

Pour in the reserved broth, cover, and bring to a simmer. Cook until the squash is tender, about 15 minutes.

Remove from heat and stir in the miso paste until fully dissolved.

Add the mussel and clam meats and season with salt.

Ladle soup into bowls. Garnish with chopped tarragon and a drizzle of tarragon oil if desired.

Blanch the tarragon in heavily salted boiling water for 10 seconds. Remove and chill under cold running water.

Place the tarragon, salt, and oil in a blender and purée until smooth.

Strain through a fine mesh strainer, pressing with the back of a spoon to extract as much oil as possible.