Preheat the grill.
Generously butter one side of each slice of bread. Add a slice or two of the Asiago cheese, Gouda and Swiss, tomato and mustard to the non-buttered side. Top with another slice of bread, buttered-side out. Lay the sandwiches on the grill and cook until brown. You might need to move the sandwich around a bit, as hot spots tend to cause the grilled cheese to cook faster in some areas than others. Flip and cook until the other side is brown and crispy, then remove from the grill. Repeat for the remaining sandwiches.