Mix Cheddar cheese, horseradish mustard, garlic, salt, pepper, and cayenne pepper together in a bowl.
Spread 1 teaspoon butter on one side of a slice of bread. Flip; spread Cheddar cheese mixture evenly over unbuttered side. Top with the other slice of bread. Spread 1 teaspoon butter on top.
Preheat a griddle over medium-high heat. Place sandwich on the griddle and press down with a spatula to flatten slightly. Cook until browned, 2 to 3 minutes. Flip and continue cooking until second side is browned, 2 to 3 minutes more.
If you can't find horseradish mustard, add 1/2 teaspoon horseradish to the filling.
A 3:1 ratio of regular-to-hot Chinese mustard works well also. My sister likes hers with sliced tomato and no horseradish. Mom likes hers with no garlic or cayenne pepper. Strictly speaking, everything in the filling is optional except the cheese, so this is a good recipe to play with.
Per Serving: 700 calories;50.1 g fat;
31.7 g carbohydrates;
36.4 g protein;
143 mg cholesterol;
1808 mg sodium.