Take brioche slices and form sandwiches with ham & cheese slices. Brush sandwiches with melted butter on both sides and grill (An electric grill, outdoor grill or cast iron grill pan can be used).
Be sure to achieve a high enough heat to achieve grill marks and cheese to melt thoroughly.
Remove from heat and allow sandwiches to rest. Combine mayo, mustard herbs and seasoning, mix to combine.
Cut sandwiches into 1/4 and serve with Tarragon mayo on the side.