1. Preheat oven to 425°F with pan inside.
2. Combine flour, salt and sugar thoroughly.
3. In a separate bowl, combine milk and eggs.
4. Combine wet and dry ingredients. Stir in cheese and onions.
5. Melt a dab of butter in each popover pan cup in the oven until it sizzles. Pour the batter into hot pan cups, 3/4 full.
6. Bake at 400°F for 15 minutes. Turn the oven down to 350°F and bake for 15 minutes. Remove and cool for 15 minutes. Serve warm.