Preheat oven to 425 degrees
Grease popover pans generously
Place the popover pans in the oven for 2 minutes to pre-heat
Meanwhile, whisk together the flour, salt, eggs, milk, and melted butter until smooth. Take out the popover pans and carefully fill them with the batter less than half full. Bake for exactly 30 minutes. Don't peak! When done, grate with gruyere (if using).