1. Preheat oven to 425°F with pan (like a muffin tin) inside oven. Combine flour, salt and sugar thoroughly. In a separate bowl, combine milk and eggs.
2. Combine wet and dry ingredients. Stir in cheese and onions. 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.