I suggest using backerboard, even if only 1/4" thick. The metal will expand and contract with the temperature changes. The fatigue on the mastic or any other material could cause failure in that bond, not to mention the integrity of the grout joints.
Schluter (http://www.schluter.co.uk/floor_trim_profiles.aspx) makes square edging in various sizes and finishes. Get the edging wide enough to hid the backer and the tile, typically 3/4" or 7/8" depending on the tile. Use mastic or another adhesive and screw the backerboard to the face of the fireplace (yes, I know it's tin, so use sheet metal screws). Adjust your trim for square and stick your tile to the backer.
Since the masonry backerboard is less a concern for expansion and contraction, your tile will stay fastened to that. If the adhesive you use to stick the backerboard to the surround fails, the screws will keep the backer in place, thus ensuring a long life for your tile.
If you're using marble or granite slabs, disregard what I just said and use clear silicone to glue them in place....
My $.02 for what its worth...
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