Cook the potatoes in salted boiling water until fork tender. (you might want to start working on your sauce while the potatoes are cooking!)
Drain and transfer to the bowl of your stand mixer equipped with the paddle (if you don't have a stand mixer, you can also use a hand mixer or a potato masher)
Add Greek yogurt, buttermilk, salt, pepper and nutmeg.
Mix on medium speed until nice and fluffy. You might have to scrape the sides a few times. When the potatoes are ready, cover the bowl with a clean towel and set aside.
To make the sauce
Drizzle a tiny little bit of olive oil in a small saucepan and heat over medium heat. Add the onion, salt and pepper and cook until fragrant and starts to turn translucent, about 3-4 minutes.
Add the flour and whisk until well combined with the onions.
Pour the milk slowly while whisking constantly and making short pauses from time to time to whisk until the sauce becomes nice and smooth, then start pouring again.
Once all the milk has been added, whisk in the Dijon mustard, dried parsley and nutmeg. Stir in clams and set aside.
For the seafood
Drizzle a medium non-stick pan with a little bit of olive oil and preheat over medium-high heat. When your pan is nice and hot, add the scallops and cook without moving for about 1 minute, until a nice golden crust forms.
Flip the scallops over using tongs and continue cooking until they get the nice crust on that side too. Remove from pan and set aside.
Add the shrimp to the pan and quickly sauté them until they just turn pink and opaque, not really much more than a minute. Remove from pan and set aside.
Add the salmon to the pan and cook until it just turns opaque, again, this shouldn't take much more than a minute or two. Add the salmon to the reserved sauce and stir it in. Reserve.
Assembly
Place a scallop in the center of each individual oven safe dish; Surround with four of the shrimp.
Cover with sauce, dividing it equally between each plate.
Give the potatoes a quick stir and place them in a large pastry bag equipped with a star tip. Pipe a border along the side of your Coquille St-Jacques.
Top with grated cheese and place in the oven for about 15-20 minutes until the top becomes nice and golden and the cheese starts to bubble.