Grate potatoes and onion on the large holes of a box grater. Splash with lemon juice and squeeze out excess water.
Mix flour, milk, egg, baking powder, and nutmeg in a large bowl. Add to the potatoes and then add the melted butter. Season with salt and pepper.
In a large skillet over medium heat, add enough peanut oil to cover the bottom of the pan. Drop in spoonfuls of batter for form small cakes and fry until golden on 1 side. Flip, and cook the same way on the other side.
Top each latke with smoked salmon, creme fraiche, and American farmed caviar.