In small bowl mix mustard, mayonnaise and tarragon. Spread on bread slices, dividing equally. For each sandwich, top 1 of the bread slices with half the Swiss Cheese and turkey. Cover with another bread slice. Melt butter in large non-stick skillet. Add sandwiches. Cook over medium heat, turning once, until bread is toasted and cheese is melted, 3-4 minutes on each side. Serve hot.