Combine the chilies, mayonnaise, and mustard in a small bowl. Spread evenly on one side of each slice of bread.
On 5 of the bread slices, place 1 slice of cheese and 2 slices of roast beef on the chili mixture. Cover with the remaining bread slices, chili mixture on the inside.
Butter the outsides of the sandwiches.
Grill the sandwiches on a griddle or skillet on medium heat until both sides are golden brown and the cheese has melted.
Notes
Mayonnaise can also be used on the outside of the bread if you prefer it to butter or margarine.