The basic French sandwich is a baguette sliced horizontally with a thin spread of Dijon mustard on one half, butter on the other half, with a slice or two of ham and thin slices of gruyere cheese.
That's fine if you're hungry, but an open-faced grilled version in a cafe is better. I had two of these in the past 10 days. Delicious lunch.
Here's how to make it: Grill an American white bread in a pan with plenty of butter, on one side only. Put a small layer of ham, then top with a generous layer of cheese. Many choices of cheese to use. Then put under a broiler until the top turns brown.
That's it. Here's one I had last week:

If you want to go more complicated, there is the famous Croque Monsieur