You can substitute the butter for olive oil in this recipe.  If necessary, move the pan around on the burner to make sure the crust browns evenly.  The roesti is best served immediately.  Great served as a brunch item.
 Ingredients:
5 tablespoons unsalted butter
1 pound red potatoes, peeled, coarsely shredded, and patted dry
1 pound celery root, peeled and shredded
2 teaspoons grated lemon zest from 1 lemon
2 tablespoons minced fresh parsley leaves
3/4 teaspoon sea salt
1/4 teaspoon ground black pepper
 
Directions:
Melt butter in 10-inch nonstick or well-seasoned cast-iron skillet over medium heat.
 
Mix shredded potatoes and celery root with salt and pepper.  Mix in lemon zest and parsley.  Spread vegetable mixture evenly; press with spatula to form compact cake.  Cook until bottom crust is golden brown, pressing on potatoes occasionally with spatula, about 15 minutes.
 
Shake skillet or use spatula to loosen roesti.  Place large plate or platter over skillet and carefully in vert.  Slide roesti back into skillet and cook until bottom is golden brown, 10 to 15 minutes longer.  Serve immediately.  Serves 4 to 6