In a large saucepan, heat 1 tbsp. EVOO over medium heat. Add the garlic and cook until lightly golden, 1 to 2 minutes. Add the potatoes and 1 1/2 cups water. Bring to a simmer, cover and cook, stirring once halfway through, until softened, about 10 minutes; drain and return to the pot.
Meanwhile, preheat a large, heavy skillet over medium-high heat. Rub the salmon with the remaining 1 tbsp. EVOO and season with salt and pepper. When the skillet is hot, add the salmon, skin side down, and cook, undisturbed, until the sin is very crisp, about 3 minutes. Lower the heat to medium and cook for 3 more minutes. Flip and cook until almost opaque at the thickest part, 1 to 2 minutes.
Meanwhile, mash the butter into the potatoes until mostly melted. Add the cheese, milk, frisee and scallions and coarsely mash; season with salt and pepper. Serve with the salmon.