"PJ Special" Grilled Cheese
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.