1. In a small bowl, mix mustard, mayonnaise and tarragon.
2. Spread on bread slices, dividing equally.
3. For each sandwich, top one of the bread slices with half the cheese and turkey.
4. Cover with another bread slice. Melt butter in large non-stick skillet.
5. Add sandwiches. Cook over medium heat until bread is toasted and cheese is melted, a few minutes on each side, until the desired brown level and crispness is attained.