Achieving a silky, lump-free béchamel is crucial; whisk continuously while adding milk. For the eggs, cook them to your preferred doneness – a slightly runny yolk is traditional for Croque Madame. When toasting the bread, aim for a light golden color before adding toppings, as it will continue to warm under the foil. Using good quality ham and Swiss cheese will significantly enhance the flavor. Don't skip the fresh nutmeg in the béchamel; it adds a subtle warmth and complexity that elevates the dish. For an extra touch, briefly broil the assembled sandwich (without the egg) to get a deeply golden, bubbly cheese before adding the egg.