Pickled mustard seeds: Pour mustard seeds, 1 1/2 cups vinegar and salt into a saucepan and allow mixture to sit for at least 1 hour and up to 12 hours.
Transfer mixture to the stove, add sugar, honey and turmeric and simmer over medium-low heat, for about 20 to 25 minutes, stirring frequently.
Remove mixture from heat and stir in remaining vinegar. Taste and adjust seasonings. Transfer to a jar and store until ready to use.
Roasted sweet potato puree: Preheat oven to 375˚F. Place sweet potatoes directly onto center rack and roast until fork tender, 30 to 40 minutes.
Remove sweet potatoes from oven and cool until able to handle.
While sweet potato cools, place a medium skillet over medium heat and add 2 tablespoons butter. Add shallots and garlic and sauté for 3 to 4 minutes.
Transfer mixture to well of blender.
Peel potatoes and add flesh into the well of a blender with shallots and garlic, discard skins.
Add heavy cream to blender and puree and season with salt and pepper. With the motor running, add remaining butter, 1 tablespoon at a time until fully incorporated and potatoes and silky smooth. Adjust seasonings.
Scallops: Pat scallops dry and season with salt and pepper. Place a large skillet over medium-high heat and add oil. Once oil is hot, add 1 tablespoons butter and scallops.
Allow scallops to sear, untouched to develop a nice, brown crust, 3 to 4 minutes.
Flip each scallop (they should pull away from the pan very easily or they are not ready yet) and sear the other side for an additional 3 to 4 minutes.
Add remaining tablespoon butter to skillet, tilt pan away from you and spoon melted butter over each scallop a few times. Remove from heat and adjust seasonings.
Serving: Spread sweet potato puree onto plates and top with scallops. Drizzle a small amount of pan juices over each scallop and top with a small spoonful of pickled mustard. Serve.