1. Grate potatoes into a large bowl.
2. Add onions and season with salt and pepper.
3. Heat a large skillet over medium to low heat and add butter and oil.
4. Add potato mixture and pat down into the pan so the bottom of the pan is completely covered and the cake is about 1 inch thick.
5. As the cake cooks, pat it down along the edges to compact it.
6. Cook for about twenty minutes, until it has formed a thick, golden crust.
7. Flip pancake by inverting it onto a large plate and then sliding it uncooked-side down back into the hot pan.
8. Cook the other side until golden and firm.
9. Slide pancake onto a serving plate and garnish with slices of smoked salmon and sour cream dressing.
10. Place the greens on top and serve.
1. Mix ingredients in a small bowl.