Peel the celeriac and rutabagas. Cut the vegetables into small, 1/2-inch pieces.
In a large cooking pot, heat a generous drizzle of olive oil over medium heat. Add the chopped vegetables and the garlic and cook over medium heat for about 2 minutes, stirring and turning the vegetables so they are coated with the oil. Add a generous amount of salt and black pepper. Add the stock and bring to a simmer. Turn the heat to low and partially cover the pan. Cook for 20 to 25 minutes, or until the vegetables are quite tender.
Turn off the heat and transfer the vegetables and broth to a blender and blend until smooth and creamy. Taste and add salt if necessary. Add the sour cream and blend again. Return to the pan and season to taste with black pepper and nutmeg. Keep warm over a very low flame, stirring occasionally.
To cook the scallops, heat the butter and olive oil in a wide, flat skillet or sauté pan over medium-high heat. When the foaming subsides, pat the scallops dry one last time and add the scallops to the skillet. Sprinkle the scallops with salt and pepper and cook for about 1 1/2 minutes. Then flip over and cook for 1 1/2 to 2 minutes on the other side, sprinkling with salt and pepper as they cook.
Remove and place on a bed of the puree and serve immediately.