Growing up, my mom made us the most delicious grilled cheese sandwiches using extra sharp white cheddar and good bread, along with a healthy slather of butter on the outsides. When she had it with us, she would always dip her grilled cheese sandwich in Dijon mustard and I thought it was the strangest thing. Until I tried it.
Now, I can't eat a grilled cheese without mustard. It's the perfect, tangy complement to the creamy, melty cheese and crusty bread. And I'm convinced the mustard takes this simple, classic sandwich up a few notches. Served with Dijon, I believe they're elegant enough for guests--add a fresh green salad and a glass of chilled white wine, and they just might believe they've landed in Paris for lunch.
The ingredients are truly so important for a sandwich this simple. Try to use fresh bakery bread if possible. Use high quality sharp cheddar, or gruyére, and don't use too much or it will overwhelm the bread and become too rich (I grate the cheese instead of slicing it because I find it melts more evenly this way). Coat the bread with butter and let it really crisp in your pan. And Grey Poupon Dijon Mustard can't be beat.