03Melt 3 tablespoons butter in same skillet over medium heat. Add potatoes, spreading out in an even layer. Cook without stirring until bottom and edges are crisp and golden brown, about 10 minutes. Slide potato pancake onto a large plate. Invert onto another large plate. Melt 1 tablespoon butter in skillet. Slide potato pancake into skillet, browned side up; cook until bottom is golden brown, about 10 minutes.
04Scatter spinach, leeks, and sausages over potato pancake, each in an even layer. Season with salt and pepper, then pour in egg mixture. Sprinkle both cheeses over.
05Bake until casserole is puffed and golden brown, 35-40 minutes. Serve warm or at room temperature.